Improved Readability

Added section headers and made some rearrangements to improve readability.
Removed discouraged '$?' usage.
Added install_program function.

install.sh

@@ -1,7 +1,33 @@
 #! /usr/bin/env bash
 
-# Grub2 Themes
+
-set  -o errexit
+# Exit Immediately if a command fails
+set -o errexit
+
+
+#
+# ──────────────────────────────────────────────────── I ──────────
+#   :::::: C O L O R S : :  :   :    :     :        :          :
+# ──────────────────────────────────────────────────────────────
+#
+
+CDEF=" \033[0m"                                     # default color
+CCIN=" \033[0;36m"                                  # info color
+CGSC=" \033[0;32m"                                  # success color
+CRER=" \033[0;31m"                                  # error color
+CWAR=" \033[0;33m"                                  # waring color
+b_CDEF=" \033[1;37m"                                # bold default color
+b_CCIN=" \033[1;36m"                                # bold info color
+b_CGSC=" \033[1;32m"                                # bold success color
+b_CRER=" \033[1;31m"                                # bold error color
+b_CWAR=" \033[1;33m"                                # bold warning color
+
+
+#
+# ────────────────────────────────────────────────────── II ──────────
+#   :::::: G L O B A L S : :  :   :    :     :        :          :
+# ────────────────────────────────────────────────────────────────
+#
 
 [  GLOBAL::CONF  ]
 {
@@ -18,17 +44,12 @@ THEME_VARIANTS=('tela' 'vimix' 'stylish' 'whitesur')
 ICON_VARIANTS=('color' 'white' 'whitesur')
 SCREEN_VARIANTS=('1080p' '2k' '4k' 'ultrawide' 'ultrawide2k')
 
-#COLORS
+
-CDEF=" \033[0m"                                     # default color
+#
-CCIN=" \033[0;36m"                                  # info color
+# ────────────────────────────────────────────────────────── III ──────────
-CGSC=" \033[0;32m"                                  # success color
+#   :::::: F U N C T I O N S : :  :   :    :     :        :          :
-CRER=" \033[0;31m"                                  # error color
+# ────────────────────────────────────────────────────────────────────
-CWAR=" \033[0;33m"                                  # waring color
+#
-b_CDEF=" \033[1;37m"                                # bold default color
-b_CCIN=" \033[1;36m"                                # bold info color
-b_CGSC=" \033[1;32m"                                # bold success color
-b_CRER=" \033[1;31m"                                # bold error color
-b_CWAR=" \033[1;33m"                                # bold warning color
 
 # echo like ... with flag type and display message colors
 prompt () {
@@ -49,7 +70,7 @@ prompt () {
 
 # Check command availability
 function has_command() {
-  command -v $1 > /dev/null
+  command -v $1 &> /dev/null #with "&>", all output will be redirected.
 }
 
 usage() {
@@ -72,7 +93,7 @@ install() {
   if [[ "$UID" -eq "$ROOT_UID" ]]; then
     clear
 
-    # Create themes directory if it didn't exist
+    # Make a themes directory if it doesn't exist
     prompt -s "\n Checking for the existence of themes directory..."
 
     [[ -d "${THEME_DIR}/${theme}" ]] && rm -rf "${THEME_DIR}/${theme}"
@@ -187,46 +208,45 @@ install() {
     updating_grub
 
     prompt -w "\n * At the next restart of your computer you will see your new Grub theme: '$theme' "
+
+  #Check if password is cached (if cache timestamp has not expired yet)
+  elif sudo -n true 2> /dev/null && echo; then #No need for "$?" ==> https://github.com/koalaman/shellcheck/wiki/SC2181
+
+    sudo "$0" -t ${theme} -i ${icon} -s ${screen}
   else
-    #Check if password is cached (if cache timestamp not expired yet)
-    sudo -n true 2> /dev/null && echo
 
-    if [[ $? == 0 ]]; then
+    #Ask for password
-      #No need to ask for password
+    if [[ -n ${tui_root_login} ]] ; then
-      sudo "$0" -t ${theme} -i ${icon} -s ${screen}
+
+      if [[ -n "${theme}" && -n "${screen}" ]]; then
+
+        sudo -S $0 -t ${theme} -i ${icon} -s ${screen} <<< ${tui_root_login}
+      fi
     else
-      #Ask for password
+
-      if [[ -n ${tui_root_login} ]] ; then
+      prompt -e "\n [ Error! ] -> Run me as root! "
-        if [[ -n "${theme}" && -n "${screen}" ]]; then
+      read -p " [ Trusted ] Specify the root password : " -t ${MAX_DELAY} -s
-          sudo -S $0 -t ${theme} -i ${icon} -s ${screen} <<< ${tui_root_login}
+
-        fi
+      if sudo -S echo <<< $REPLY 2> /dev/null && echo; then
+
+        #Correct password, use with sudo's stdin
+        sudo -S "$0" -t ${theme} -i ${icon} -s ${screen} <<< ${REPLY}
       else
-        prompt -e "\n [ Error! ] -> Run me as root! "
-        read -p " [ Trusted ] Specify the root password : " -t ${MAX_DELAY} -s
 
-        sudo -S echo <<< $REPLY 2> /dev/null && echo
+        #block for 3 seconds before allowing another attempt
-
+        sleep 3
-        if [[ $? == 0 ]]; then
+        prompt -e "\n [ Error! ] -> Incorrect password!\n"
-          #Correct password, use with sudo's stdin
+        exit 1
-          sudo -S "$0" -t ${theme} -i ${icon} -s ${screen} <<< ${REPLY}
-        else
-          #block for 3 seconds before allowing another attempt
-          sleep 3
-          prompt -e "\n [ Error! ] -> Incorrect password!\n"
-          exit 1
-        fi
       fi
     fi
- fi
+  fi
 }
 
 run_dialog() {
   if [[ -x /usr/bin/dialog ]]; then
     if [[ "$UID" -ne "$ROOT_UID"  ]]; then
       #Check if password is cached (if cache timestamp not expired yet)
-      sudo -n true 2> /dev/null && echo
+      if sudo -n true 2> /dev/null && echo; then
-
-      if [[ $? == 0 ]]; then
         #No need to ask for password
         sudo $0
       else
@@ -237,9 +257,7 @@ run_dialog() {
         --passwordbox  "require root permission" 8 50 \
         --output-fd 1 )
 
-        sudo -S echo <<< $tui_root_login 2> /dev/null && echo
+        if sudo -S echo <<< $tui_root_login 2> /dev/null && echo; then
-
-        if [[ $? == 0 ]]; then
           #Correct password, use with sudo's stdin
           sudo -S "$0" <<< $tui_root_login
         else
@@ -324,20 +342,30 @@ updating_grub() {
   prompt -s "\n * All done!"
 }
 
+function install_program () {
+
+  if has_command zypper; then
+    
+    zypper in "$@"
+  elif has_command apt-get; then
+    
+    apt-get install "$@"
+  elif has_command dnf; then
+    
+    dnf install -y "$@"
+  elif has_command yum; then
+    
+    yum install "$@"
+  elif has_command pacman; then
+    
+    pacman -S --noconfirm "$@"
+  fi
+}
+
 install_dialog() {
   if [ ! "$(which dialog 2> /dev/null)" ]; then
     prompt -w "\n 'dialog' need to be installed for this shell"
-    if has_command zypper; then
+    install_program "dialog"
-      sudo zypper in dialog
-    elif has_command apt-get; then
-      sudo apt-get install dialog
-    elif has_command dnf; then
-      sudo dnf install -y dialog
-    elif has_command yum; then
-      sudo yum install dialog
-    elif has_command pacman; then
-      sudo pacman -S --noconfirm dialog
-    fi
   fi
 }
 
@@ -374,9 +402,7 @@ remove() {
 
   else
     #Check if password is cached (if cache timestamp not expired yet)
-    sudo -n true 2> /dev/null && echo
+    if sudo -n true 2> /dev/null && echo; then
-
-    if [[ $? == 0 ]]; then
       #No need to ask for password
       sudo "$0" "${PROG_ARGS[@]}"
     else
@@ -384,9 +410,7 @@ remove() {
       prompt -e "\n [ Error! ] -> Run me as root! "
       read -p " [ Trusted ] Specify the root password : " -t ${MAX_DELAY} -s
 
-      sudo -S echo <<< $REPLY 2> /dev/null && echo
+      if sudo -S echo <<< $REPLY 2> /dev/null && echo; then
-
-      if [[ $? == 0 ]]; then
         #Correct password, use with sudo's stdin
         sudo -S "$0" "${PROG_ARGS[@]}" <<< $REPLY
       else
@@ -404,9 +428,8 @@ dialog_installer() {
   if [[ ! -x /usr/bin/dialog ]];  then
     if [[ $UID -ne $ROOT_UID ]];  then
       #Check if password is cached (if cache timestamp not expired yet)
-      sudo -n true 2> /dev/null && echo
 
-      if [[ $? == 0 ]]; then
+      if sudo -n true 2> /dev/null && echo; then
         #No need to ask for password
         exec sudo $0
       else
@@ -414,9 +437,7 @@ dialog_installer() {
         prompt -e "\n [ Error! ] -> Run me as root! "
         read -p " [ Trusted ] Specify the root password : " -t ${MAX_DELAY} -s
 
-        sudo -S echo <<< $REPLY 2> /dev/null && echo
+        if sudo -S echo <<< $REPLY 2> /dev/null && echo; then
-
-        if [[ $? == 0 ]]; then
           #Correct password, use with sudo's stdin
           sudo $0 <<< $REPLY
         else
@@ -433,6 +454,13 @@ dialog_installer() {
   install "${theme}" "${icon}" "${screen}"
 }
 
+
+#
+# ────────────────────────────────────────────────────────────────────────── IV ──────────
+#   :::::: A R G U M E N T   H A N D L I N G : :  :   :    :     :        :          :
+# ────────────────────────────────────────────────────────────────────────────────────
+#
+
 while [[ $# -gt 0 ]]; do
   PROG_ARGS+=("${1}")
   dialog='false'
@@ -465,7 +493,7 @@ while [[ $# -gt 0 ]]; do
             themes+=("${THEME_VARIANTS[3]}")
             shift
             ;;
-          -*|--*)
+          -*) # "-*" overrides "--*"
             break
             ;;
           *)
@@ -492,7 +520,7 @@ while [[ $# -gt 0 ]]; do
             icons+=("${ICON_VARIANTS[2]}")
             shift
             ;;
-          -*|--*)
+          -*)
             break
             ;;
           *)
@@ -527,7 +555,7 @@ while [[ $# -gt 0 ]]; do
             screens+=("${SCREEN_VARIANTS[4]}")
             shift
             ;;
-          -*|--*)
+          -*)
             break
             ;;
           *)
@@ -550,6 +578,13 @@ while [[ $# -gt 0 ]]; do
   esac
 done
 
+
+#
+# ──────────────────────────────────────────────── V ──────────
+#   :::::: M A I N : :  :   :    :     :        :          :
+# ──────────────────────────────────────────────────────────
+#
+
 # Show terminal user interface for better use
 if [[ "${dialog:-}" == 'false' ]]; then
   if [[ "${remove:-}" != 'true' ]]; then