Add xresources theming

Xresources processor is very customized and not useable for default
Xresources deployment. It depends on Xresources being available as an
XDG-compliant directory in $XDG_CONFIG_HOME/xresources.

Future work on this processor should start from the default xresources
deployment and allow customization to change it (e.g. through stylerrc
file, setting custom options and paths per processor)

theme_xresources

@@ -0,0 +1,98 @@
+#!/usr/bin/env bash
+readonly dependency=("binaryplease/base16-xresources")
+
+path="$1"
+package="$2"
+theme="$3"
+permanent="$4"
+
+file_exists() {
+  if [[ -f "$1" ]]; then
+    true
+  else
+    false
+  fi
+}
+# check for existence of pattern $2 in file $1
+line_exists() {
+  local file="$1"
+  local line="$2"
+
+  if ! file_exists "$file" || ! grep -qe "$line" "$file"; then
+    false
+  else
+    true
+  fi
+}
+# prepare newline at eof to make adding newlines easier
+eol_exists_or_append() {
+  local file="$1"
+  local eof
+  eof=$(tail -c 1 "$file")
+
+  if [ -n "$eof" ]; then
+    printf "\\n" >>"$file"
+  fi
+}
+# append line $2 to file $1
+line_exists_or_append() {
+  local file="$1"
+  local line="$2"
+  local new="${3:-$2}"
+
+  if ! line_exists "$file" "$line"; then
+    eol_exists_or_append "$file"
+    echo "$new" >>"$file"
+  fi
+}
+check_tfile() {
+  if ! file_exists "$tfile"; then
+    dbg_msg="$dbg_msg theme $theme for xresources not found.\n"
+    exit 1
+  fi
+}
+
+theme_xresources() {
+  dbg_msg="XRESOURCES: $package - "
+  tfile="$path/$package/xresources/base16-$theme.Xresources"
+
+  check_tfile
+
+  if [[ "$permanent" == "true" ]]; then set_xresources_theme; fi
+  switch_xresources_theme
+
+  [[ "$DEBUG" == true ]] && echo "$dbg_msg"
+}
+
+switch_xresources_theme() {
+  local x_dir="${XDG_CONFIG_HOME:-/$HOME/.config}/xresources"
+
+  xrdb -load "$x_dir/Xresources"
+}
+
+set_xresources_theme() {
+  local x_dir="${XDG_CONFIG_HOME:-/$HOME/.config}/xresources"
+  local x_optdir="$x_dir/Xresources.d"
+
+  if [[ -d "$x_optdir" ]]; then
+    cat "$tfile" >"$x_optdir/colorscheme"
+
+    call_from_config "$x_dir"
+
+    dbg_msg="$dbg_msg Set theme $theme\n"
+  fi
+}
+
+call_from_config() {
+  local x_dir="$1"
+
+  if file_exists "$x_dir/Xresources"; then
+    line_exists_or_append "$x_dir/Xresources" '#include "Xresources.d/colorscheme"'
+  fi
+}
+
+if printf '%s\n' "${dependency[@]}" | grep -q -P "^$package$"; then
+  theme_xresources
+else
+  printf "Processor does not work for %s, please use another." "$package"
+fi