Styles/GTK: add selection (#841)

Author: danirabbit

lib/Styles/Gtk/Index.scss

@@ -11,6 +11,7 @@
 @import 'Scale.scss';
 @import 'ScrolledWindow.scss';
 @import 'Scrollbar.scss';
+@import 'Selection.scss';
 @import 'ShortcutsWindow.scss';
 @import 'Slider.scss';
 @import 'Spinner.scss';

lib/Styles/Gtk/Selection.scss

@@ -0,0 +1,9 @@
+selection {
+    background-color: #{'@selected_bg_color'};
+    color: #{'@selected_fg_color'};
+
+    &:backdrop {
+        background-color: scale-color($fg-color, $alpha: -90%);
+        color: inherit;
+    }
+}

lib/Styles/_exported.scss

@@ -3,3 +3,6 @@
 // these exported GtkCSS variables should be considered public API.
 
 @define-color fg_color #{"" + $fg-color};
+
+@define-color selected_bg_color #{'alpha(@accent_color, 0.25)'};
+@define-color selected_fg_color #{'shade(@accent_color, 0.5)'};