Cleanup unused variables from color themes

closes #7121

app/assets/stylesheets/color_themes/dark_green/_style.scss

@@ -3,16 +3,11 @@ $brand-primary: #009900;
 
 /* Shades */
 $main-color-essence: fade-out($brand-primary, 0.8);
-$main-color-light: lighten($brand-primary, 10%);
 $main-color-dark: darken($brand-primary, 20%);
 
 /* Bootstrap Variables */
 $btn-primary-bg: $brand-primary;
 $link-color: $brand-primary;
-
-/* Custom Variables */
-$header-background-color: $main-color-dark;
 $navbar-inverse-bg: $main-color-dark;
-$header-search-color: lighten($header-background-color, 10%);
 
-@import "color_themes/color_theme_override"
+@import 'color_themes/color_theme_override';

app/assets/stylesheets/color_themes/egyptian_blue/_style.scss

@@ -3,16 +3,11 @@ $brand-primary: #1034A6;
 
 /* Shades */
 $main-color-essence: fade-out($brand-primary, 0.8);
-$main-color-light: lighten($brand-primary, 10%);
 $main-color-dark: darken($brand-primary, 15%);
 
 /* Bootstrap Variables */
 $btn-primary-bg: $brand-primary;
 $link-color: $brand-primary;
-
-/* Custom Variables */
-$header-background-color: $main-color-dark;
 $navbar-inverse-bg: $main-color-dark;
-$header-search-color: lighten($header-background-color, 15%);
 
-@import "color_themes/color_theme_override"
+@import 'color_themes/color_theme_override';

app/assets/stylesheets/color_themes/magenta/_style.scss

@@ -3,16 +3,11 @@ $brand-primary: #FF00FF;
 
 /* Shades */
 $main-color-essence: fade-out($brand-primary, 0.8);
-$main-color-light: lighten($brand-primary, 10%);
 $main-color-dark: darken($brand-primary, 30%);
 
 /* Bootstrap Variables */
 $btn-primary-bg: darken($brand-primary, 5%);
 $link-color: $brand-primary;
-
-/* Custom Variables */
-$header-background-color: $main-color-dark;
 $navbar-inverse-bg: $main-color-dark;
-$header-search-color: lighten($header-background-color, 10%);
 
-@import "color_themes/color_theme_override"
+@import 'color_themes/color_theme_override';

app/assets/stylesheets/color_themes/original/_style.scss

-/* Main color(s) */
-$main-color: #585858;
-
-/* Shades */
-$main-color-dark: darken($main-color, 15%);
-
-/* Variables */
-$header-background-color: $main-color-dark;

app/assets/stylesheets/color_themes/original/desktop.scss

-@import "style";
 @import "application";

app/assets/stylesheets/color_themes/original/mobile.scss

-@import "style";
 @import "mobile/mobile";

app/assets/stylesheets/color_themes/original_white/_style.scss

 // Main color(s)
-$main-color: #585858;
 $background: #fff;
 
-// Shades
-$main-color-dark: darken($main-color, 15%);
-
 // Variables
-$header-background-color: $main-color-dark;
 $main-background: $background;
 $sidebars-background: $background;
 $card-shadow: none;