Add css var support for dark theme

Author: thornbill

src/styles/_mixins.scss

@@ -10,3 +10,11 @@
         #{$property}: max($max-value, $default);
     }
 }
+
+/**
+ * Helper mixin to add properties using css variables with fallback values.
+ */
+@mixin var($property, $variable, $default, $important: false) {
+    #{$property}: $default if($important, !important, null);
+    #{$property}: var($variable) if($important, !important, null);
+}

src/styles/librarybrowser.scss

@@ -1017,16 +1017,6 @@ div.itemDetailGalleryLink.defaultCardBackground {
     }
 }
 
-.emby-button.detailFloatingButton {
-    font-size: 1.4em;
-    margin-right: 0.5em !important;
-    color: rgba(255, 255, 255, 0.76);
-}
-
-.emby-button.detailFloatingButton .material-icons {
-    font-size: 3.5em;
-}
-
 @media all and (max-width: 62.5em) {
     .parentName {
         margin-bottom: 0;