Implement custom comment brightness customization

> Description

Added a accessibility theme customization to allow users to set the
brightness of comments with percentage adjustments from 0% - 20% where 0
is the default nord3 comment color. When a invalid value is
passed (< 0 || > 20 || nil) the default nord3 color is used.

The customization is available through the nord-comment-brightness variable.

This is a optional customization to provide a way for users to adjust
the color to fit their needs: It is a port of the reference
implementation from the Nord Atom UI project. The values are calculated
using the LESSCSS lighten function to ensure full interoperability with
other port projects that providing this theme feature.

> Usage

Example to increase the comment color brightness by 15%:
  (setq nord-comment-brightness 15)

References:
nordtheme/atom-syntax#47
http://lesscss.org/functions/#color-operations-lighten

GH-43

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)
+    - [Custom Comment Brightness](#custom-comment-brightness)
   - [Package Support](#package-support)
     - [Syntax Packages](#syntax-packages)
     - [UI Packages](#ui-packages)
@@ -71,6 +73,54 @@ 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!
+
+### Custom Comment Brightness
+
+This customization allows to define a custom comment color brightness with percentage adjustments from *0* - *20*. It is a way to provide a way for users to easily adjust the comment color to fit their needs without overriding specific faces individually.
+
+It can be enabled by adding the `nord-comment-brightness` variable to a number between `1` and `20` in your `init.el`:
+
+```lisp
+(setq nord-comment-brightness 15)
+```
+
+To adhere to the Nord style guide this option uses `nord3` by default and applied as fallback when the variable is assigned a invalid value.
+
+This customization is a port of the reference implementation from the [Nord Atom Syntax][nord-atom-syntax-gh-47] project. The values are calculated using the LESSCSS [`lighten`][lesscss-doc-lighten] function to ensure full interoperability with other port projects that providing this theme feature.
+
+| Increased by | Calculated value |
+| --- | --- |
+| 0%  (default) | `nord3` |
+| 1% | `#4e586d` |
+| 2% | `#505b70` |
+| 3% | `#525d73` |
+| 4% | `#556076` |
+| 5% | `#576279` |
+| 6% | `#59647c` |
+| 7% | `#5b677f` |
+| 8% | `#5d6982` |
+| 9% | `#5f6c85` |
+| 10% | `#616e88` |
+| 11% | `#63718b` |
+| 12% | `#66738e` |
+| 13% | `#687591` |
+| 14% | `#6a7894` |
+| 15% | `#6d7a96` |
+| 16% | `#6f7d98` |
+| 17% | `#72809a` |
+| 18% | `#75829c` |
+| 19% | `#78859e` |
+| 20% | `#7b88a1` |
+
+<align="center">
+<strong>Default comment brightness</strong><br>
+<img src="https://raw.githubusercontent.com/arcticicestudio/nord-emacs/develop/assets/scrot-custom-comment-brightness-java-default.png"/><br><img src="https://raw.githubusercontent.com/arcticicestudio/nord-emacs/develop/assets/scrot-custom-comment-brightness-js-default.png"/><br>
+<strong>Increased comment brightness by 15%</strong><br>
+<img src="https://raw.githubusercontent.com/arcticicestudio/nord-emacs/develop/assets/scrot-custom-comment-brightness-java-15percent.png"/><br><img src="https://raw.githubusercontent.com/arcticicestudio/nord-emacs/develop/assets/scrot-custom-comment-brightness-js-15percent.png"/></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).
@@ -118,3 +168,6 @@ Please report issues/bugs, feature requests and suggestions for improvements to
 <p align="center"> <img src="http://arcticicestudio.com/favicon.ico" width=16 height=16/> Copyright &copy; 2017 Arctic Ice Studio</p>
 
 <p align="center"><a href="http://www.apache.org/licenses/LICENSE-2.0"><img src="https://img.shields.io/badge/License-Apache_2.0-5E81AC.svg?style=flat-square"/></a> <a href="https://creativecommons.org/licenses/by-sa/4.0"><img src="https://img.shields.io/badge/License-CC_BY--SA_4.0-5E81AC.svg?style=flat-square"/></a></p>
+
+[lesscss-doc-lighten]: http://lesscss.org/functions/#color-operations-lighten
+[nord-atom-syntax-gh-47]: https://github.com/arcticicestudio/nord-atom-syntax/issues/47

nord-theme.el

@@ -42,6 +42,28 @@
 
 (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-comment-brightness 0
+  "Allows to define a custom comment color brightness with percentage adjustments from 0% - 20%.
+  The value must be greater or equal to 0 and less or equal to 20, otherwise the default 'nord3' color is used."
+  :type 'integer
+  :group 'nord)
+
+(setq nord-brightened-comments '("#4c566a" "#4e586d" "#505b70" "#525d73" "#556076" "#576279" "#59647c" "#5b677f" "#5d6982" "#5f6c85" "#616e88" "#63718b" "#66738e" "#687591" "#6a7894" "#6d7a96" "#6f7d98" "#72809a" "#75829c" "#78859e" "#7b88a1"))
+
+(defun brightened-comment-color (percent)
+  "Returns the brightened comment color for the given percent.
+  The value must be greater or equal to 0 and less or equal to 20, otherwise the default 'nord3' color is used."
+  (if (and (integerp percent) 
+          (>= percent 0) 
+          (<= percent 20)) 
+    (nth percent nord-brightened-comments)
+    (nth 0 nord-brightened-comments)))
+
 ;;;; Color Constants
 (let ((class '((class color) (min-colors 89)))
   (nord0 (if (display-graphic-p) "#2E3440" nil))
@@ -63,7 +85,7 @@
   (nord-annotation (if (display-graphic-p) "#D08770" "brightyellow"))
   (nord-attribute (if (display-graphic-p) "#8FBCBB" "cyan"))
   (nord-class (if (display-graphic-p) "#8FBCBB" "cyan"))
-  (nord-comment (if (display-graphic-p) "#4C566A" "brightblack"))
+  (nord-comment (if (display-graphic-p) (brightened-comment-color nord-comment-brightness) "brightblack"))
   (nord-escape (if (display-graphic-p) "#D08770" "brightyellow"))
   (nord-method (if (display-graphic-p) "#88C0D0" "brightcyan"))
   (nord-keyword (if (display-graphic-p) "#81A1C1" "blue"))
@@ -88,10 +110,10 @@
     `(error ((,class (:foreground ,nord11 :weight bold))))
     `(escape-glyph ((,class (:foreground ,nord12))))
     `(font-lock-builtin-face ((,class (:foreground ,nord9))))
-    `(font-lock-comment-face ((,class (:foreground ,nord3))))
-    `(font-lock-comment-delimiter-face ((,class (:foreground ,nord3))))
+    `(font-lock-comment-face ((,class (:foreground ,nord-comment))))
+    `(font-lock-comment-delimiter-face ((,class (:foreground ,nord-comment))))
     `(font-lock-constant-face ((,class (:foreground ,nord9))))
-    `(font-lock-doc-face ((,class (:foreground ,nord3))))
+    `(font-lock-doc-face ((,class (:foreground ,nord-comment))))
     `(font-lock-function-name-face ((,class (:foreground ,nord8))))
     `(font-lock-keyword-face ((,class (:foreground ,nord9))))
     `(font-lock-negation-char-face ((,class (:foreground ,nord9))))
@@ -142,7 +164,7 @@
     `(custom-button-pressed-unraised ((,class (:background ,nord4 :foreground ,nord0 :box (:line-width 2 :color ,nord4 :style sunken-button)))))
     `(custom-button-unraised ((,class (:background ,nord0 :foreground ,nord8 :box (:line-width 2 :color ,nord4 :style sunken-button)))))
     `(custom-changed ((,class (:foreground ,nord13))))
-    `(custom-comment ((,class (:foreground ,nord3))))
+    `(custom-comment ((,class (:foreground ,nord-comment))))
     `(custom-comment-tag ((,class (:foreground ,nord7))))
     `(custom-documentation ((,class (:foreground ,nord4))))
     `(custom-group-tag ((,class (:foreground ,nord8 :weight bold))))
@@ -323,7 +345,7 @@
     `(js2-jsdoc-html-tag-name ((,class (:foreground ,nord9))))
     `(js2-external-variable ((,class (:foreground ,nord4))))
     `(js2-function-param ((,class (:foreground ,nord4))))
-    `(js2-jsdoc-value ((,class (:foreground ,nord3))))
+    `(js2-jsdoc-value ((,class (:foreground ,nord-comment))))
     `(js2-jsdoc-tag ((,class (:foreground ,nord7))))
     `(js2-jsdoc-type ((,class (:foreground ,nord7))))
     `(js2-private-member ((,class (:foreground ,nord4))))
@@ -348,7 +370,7 @@
     `(js3-warning-face ((,class (:foreground ,nord13))))
 
     ;; > Markdown
-    `(markdown-blockquote-face ((,class (:foreground ,nord3))))
+    `(markdown-blockquote-face ((,class (:foreground ,nord-comment))))
     `(markdown-bold-face ((,class (:inherit bold))))
     `(markdown-header-face-1 ((,class (:foreground ,nord8))))
     `(markdown-header-face-2 ((,class (:foreground ,nord8))))