settings: Fix background-color of save-discard widget in dark theme.

The correct background-color for buttons of save-discard widget
was not being applied and instead almost transparent color was
applied to dark-theme CSS rules. This commit adds ID to the
selector such that CSS in app_components.css is preferred over
dark-theme CSS.

static/styles/app_components.css

@@ -740,7 +740,8 @@ div.overlay {
     }
 }
 
-.save-button-controls {
+#stream_settings .save-button-controls,
+#settings_page .save-button-controls {
     display: inline;
     margin-left: 15px;