Implement uniform mode lines customization

> Description

Added a theme customization to allow users to enable uniform activate-
and inactive mode lines using "nord3" as background.

The default is left to "nord1" as background color.

The customization is available through the "nord-uniform-mode-lines"
variable of type "boolean".

> Usage

  (setq nord-uniform-mode-lines t)

Related to #32

GH-47

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)
+    - [Uniform Mode Lines](#uniform-mode-lines)
   - [Package Support](#package-support)
     - [Syntax Packages](#syntax-packages)
     - [UI Packages](#ui-packages)
@@ -71,6 +73,22 @@ 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!
+
+### Uniform Mode Lines
+
+Enables uniform activate- and inactive mode lines using `nord3` as background.
+
+It can be activated by setting the `nord-uniform-mode-lines` variable to `t` in your `init.el`:
+
+```lisp
+(setq nord-uniform-mode-lines t)
+```
+
+<align="center"><strong>Default mode lines</strong><br><img src="https://raw.githubusercontent.com/arcticicestudio/nord-emacs/develop/assets/scrot-customization-uniform-mode-lines-default.png"/><br><strong>Uniform mode lines</strong><br><img src="https://raw.githubusercontent.com/arcticicestudio/nord-emacs/develop/assets/scrot-customization-uniform-mode-lines.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).

nord-theme.el

@@ -42,6 +42,16 @@
 
 (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-uniform-mode-lines nil
+  "Enables uniform activate- and inactive mode lines using 'nord3' as background."
+  :type 'boolean
+  :group 'nord)
+
 ;;;; Color Constants
 (let ((class '((class color) (min-colors 89)))
   (nord0 (if (display-graphic-p) "#2E3440" nil))
@@ -74,7 +84,8 @@
   (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-uniform-mode-lines-background (if nord-uniform-mode-lines "#4C566A" "#3B4252")))
 
 ;;;; +------------+
 ;;;; + Core Faces +
@@ -181,10 +192,10 @@
     `(message-separator ((,class (:inherit shadow))))
     `(minibuffer-prompt ((,class (:foreground ,nord8 :weight bold))))
     `(mm-command-output ((,class (:foreground ,nord8))))
-    `(mode-line ((,class (:foreground ,nord4 :background ,nord2))))
+    `(mode-line ((,class (:foreground ,nord8 :background ,nord3))))
     `(mode-line-buffer-id ((,class (:weight bold))))
     `(mode-line-highlight ((,class (:inherit highlight))))
-    `(mode-line-inactive ((,class (:foreground ,nord4 :background ,nord1))))
+    `(mode-line-inactive ((,class (:foreground ,nord4 :background ,nord-uniform-mode-lines-background))))
     `(next-error ((,class (:inherit error))))
     `(nobreak-space ((,class (:foreground ,nord3))))
     `(outline-1 ((,class (:foreground ,nord8 :weight bold))))