Opt-in to light and dark UI controls (#95)

This ensures that e.g. scrollbars are painted dark when using the dark mode.

Author: khlr

src/resources/css/samltrace.css

@@ -1,4 +1,5 @@
 :root {
+  color-scheme: light dark;
   --color-text: #737373;
   --color-text-active: white;
   --color-background: white;