Reuse existing SCSS variables

Signed-off-by: Rémy Coutable <remy@rymai.me>

app/assets/stylesheets/framework/typography.scss

@@ -271,9 +271,9 @@ h1, h2, h3, h4 {
 }
 
 .idiff.deletion {
-  background: $gl-idiff-deletion;
+  background: $line-removed-dark;
 }
 
 .idiff.addition {
-  background: $gl-idiff-addition;
+  background: $line-added-dark;
 }

app/assets/stylesheets/framework/variables.scss

@@ -178,8 +178,6 @@ $table-border-gray: #f0f0f0;
 $line-target-blue: #eaf3fc;
 $line-select-yellow: #fcf8e7;
 $line-select-yellow-dark: #f0e2bd;
-$gl-idiff-deletion: #f8cbcb;
-$gl-idiff-addition: #a6f3a6;
 
 /*
  * Fonts

lib/gitlab/diff/inline_diff_marker.rb

@@ -28,8 +28,18 @@ module Gitlab
 
         # Mark each range
         marker_ranges.each_with_index do |range, index|
-          before_content = markdown ? "{#{MARKDOWN_SYMBOLS[mode]}" : "<span class='#{html_class_names(marker_ranges, mode, index)}'>"
-          after_content = markdown ? "#{MARKDOWN_SYMBOLS[mode]}}" : "</span>"
+          before_content =
+            if markdown
+              "{#{MARKDOWN_SYMBOLS[mode]}"
+            else
+              "<span class='#{html_class_names(marker_ranges, mode, index)}'>"
+            end
+          after_content =
+            if markdown
+              "#{MARKDOWN_SYMBOLS[mode]}}"
+            else
+              "</span>"
+            end
           offset = insert_around_range(rich_line, range, before_content, after_content, offset)
         end