css: Add focus ring custom properties

The values were mostly copied from libadwaita's, but with the
intention to easily add focus rings to custom widgets if desired.

src/theme/style.css

@@ -2,6 +2,12 @@
 
 :root {
   --insensitive-fg-color: color-mix(in srgb, var(--window-fg-color) 50%, transparent);
+
+  --focus-ring-transition:
+    outline-color 200ms cubic-bezier(0.25, 0.46, 0.45, 0.94),
+    outline-width 200ms cubic-bezier(0.25, 0.46, 0.45, 0.94),
+    outline-offset 200ms cubic-bezier(0.25, 0.46, 0.45, 0.94);
+  --focus-ring-color: rgb(from var(--accent-color) r g b / var(--dim-opacity));
 }
 
 /* Header labels */