Add: Support for selection colors on custom theme.

resources/css/common.css

@@ -1027,11 +1027,13 @@ button.context-menu-option {
 }
 
 ::selection {
-  background: #338FFF;
+  background: var(--ls-block-highlight-color);
+  color: var(--ls-primary-text-color)
 }
 
 ::-moz-selection {
-  background: #338FFF;
+  background: var(--ls-block-highlight-color);
+  color: var(--ls-primary-text-color)
 }
 
 .dropdown-overflow-auto {