Add global CSS files

They will receive styles to be loaded globally.

src/handy-fallback.css

+/*
+ * This contains fallback styles. Style goes here when we are happy to let
+ * themes use their own style, but we still need a good default in case they
+ * don't.
+ *
+ * It will be loaded with the GTK_STYLE_PROVIDER_PRIORITY_FALLBACK priority,
+ * meaning its priority is lower than themes, settings, applications, and user
+ * provided styles.
+ */

src/handy-override.css

+/*
+ * This contains styles overriding the themes. Style goes here when we want a
+ * very specific style which collides with common themes, and we need to
+ * override what they do.
+ *
+ * It will be loaded with the HDY_STYLE_PROVIDER_PRIORITY priority,
+ * which corresponds to GTK_STYLE_PROVIDER_PRIORITY_SETTINGS + 1, meaning its
+ * priority is higher than themes and settings, but lower than applications and
+ * user provided styles.
+ */

src/handy.gresources.xml

@@ -25,5 +25,7 @@
     <file>hdy-view-switcher.css</file>
     <file>hdy-view-switcher-bar-box.css</file>
     <file>hdy-view-switcher-button.css</file>
+    <file compressed="true">handy-fallback.css</file>
+    <file compressed="true">handy-override.css</file>
   </gresource>
 </gresources>