Implement region highlight style customization

> Description

Added a theme customization to allow users to set a style for the region
highlighting either to "snowstorm" or "frost", providing a way for users
to adjust the colors to fit their needs.

The default is left to "nord2" as background- and no specific foreground
color.

The customization is available through the "nord-region-highlight"
variable of type "string".

> Usage

Example using the "snowstorm" style:
  (setq nord-region-highlight "snowstorm")

Resolves #32

GH-45

README.md

@@ -20,6 +20,8 @@ Nord Emacs is a 16 colorspace theme build to run in GUI- and terminal mode with
       - [Package.el](#package-el)
     - [Activation](#activation)
   - [Features](#features)
+  - [Customization](#customization)
+    - [Region Highlight Style](#region-highlight-style)
   - [Package Support](#package-support)
     - [Syntax Packages](#syntax-packages)
     - [UI Packages](#ui-packages)
@@ -71,6 +73,24 @@ or change it on-the-fly by running <kbd>M-x</kbd> `load-theme` <kbd>RET</kbd> `n
 
 <p align="center"><strong>Colors of selected code can still be easily recognized.</strong><br><img src="https://raw.githubusercontent.com/arcticicestudio/nord-emacs/develop/assets/scrcast-feature-selection.gif"/></p>
 
+## Customization
+
+All customizations need to be set before `load-theme` is invoked for Nord and require a restart of Emacs when changed!
+
+### Region Highlight Style
+
+Allows to set a style for the region highlight based on the Nord components, either to `snowstorm` or `frost`.
+
+To adhere to the Nord style guide this option uses `nord2` as background- and no specific foreground color.
+
+It can be enabled by setting the `nord-region-highlight` variable to the desired style `snowstorm` or `frost` in your `init.el`:
+
+```lisp
+(setq nord-region-highlight "snowstorm")
+```
+
+<align="center"><strong><code>snowstorm</code> region highlight style</strong><br><img src="https://raw.githubusercontent.com/arcticicestudio/nord-emacs/develop/assets/scrot-customization-region-highlight-style-snowstorm.png"/><br><img src="https://raw.githubusercontent.com/arcticicestudio/nord-emacs/develop/assets/scrcast-customization-region-highlight-style-snowstorm.gif"/><br><strong><code>frost</code> region highlight style</strong><br><img src="https://raw.githubusercontent.com/arcticicestudio/nord-emacs/develop/assets/scrot-customization-region-highlight-style-frost.png"/><br><img src="https://raw.githubusercontent.com/arcticicestudio/nord-emacs/develop/assets/scrcast-customization-region-highlight-style-frost.gif"/><br><strong>default region highlight style</strong><br><img src="https://raw.githubusercontent.com/arcticicestudio/nord-emacs/develop/assets/scrot-customization-region-highlight-style-default.png"/><br><img src="https://raw.githubusercontent.com/arcticicestudio/nord-emacs/develop/assets/scrcast-customization-region-highlight-style-default.gif"/></p>
+
 ## Package Support
 Nord Emacs provides support for many third-party syntax- and the UI packages.  
 Detailed descriptions for supported packages can be found in the [project wiki](https://github.com/arcticicestudio/nord-emacs/wiki).

nord-theme.el

@@ -42,6 +42,19 @@
 
 (deftheme nord "An arctic, north-bluish clean and elegant theme")
 
+(defgroup nord nil
+  "Nord theme customizations.
+  The theme has to be reloaded after changing anything in this group."
+  :group 'faces)
+
+(defcustom nord-region-highlight nil
+  "Allows to set a region highlight style based on the Nord components.
+  Valid styles are
+    - 'snowstorm' - Uses 'nord0' as foreground- and 'nord4' as background color
+    - 'frost' - Uses 'nord0' as foreground- and 'nord8' as background color"
+  :type 'string
+  :group 'nord)
+
 ;;;; Color Constants
 (let ((class '((class color) (min-colors 89)))
   (nord0 (if (display-graphic-p) "#2E3440" nil))
@@ -74,7 +87,13 @@
   (nord-regexp (if (display-graphic-p) "#EBCB8B" "yellow"))
   (nord-string (if (display-graphic-p) "#A3BE8C" "green"))
   (nord-tag (if (display-graphic-p) "#81A1C1" "blue"))
-  (nord-variable (if (display-graphic-p) "#D8DEE9" "#D8DEE9")))
+  (nord-variable (if (display-graphic-p) "#D8DEE9" "#D8DEE9"))
+  (nord-region-highlight-foreground (if (or 
+    (string= nord-region-highlight "frost") 
+    (string= nord-region-highlight "snowstorm")) "#2E3440" nil))
+  (nord-region-highlight-background (if 
+    (string= nord-region-highlight "frost") "#88C0D0" 
+      (if (string= nord-region-highlight "snowstorm") "#D8DEE9" "#434C5E"))))
 
 ;;;; +------------+
 ;;;; + Core Faces +
@@ -210,7 +229,7 @@
     `(package-status-installed ((,class (:foreground ,nord7 :weight bold))))
     `(package-status-unsigned ((,class (:underline ,nord13))))
     `(query-replace ((,class (:foreground ,nord8 :background ,nord2))))
-    `(region ((,class (:background ,nord2))))
+    `(region ((,class (:foreground ,nord-region-highlight-foreground :background ,nord-region-highlight-background))))
     `(scroll-bar ((,class (:background ,nord3))))
     `(secondary-selection ((,class (:background ,nord2))))
     `(show-paren-match-face ((,class (:foreground ,nord0 :background ,nord8))))