feat(themes/prettylights): opt in to color-scheme property

Adopt theme colors based on the `color-scheme` property. Default is still the user's color scheme preference with the possibility to overwrite via the `color-scheme` property.

https://developer.mozilla.org/en-US/docs/Web/CSS/color-scheme

Author: andreruffert

src/themes/prettylights.css

@@ -25,6 +25,7 @@
     --prettylights-string-regexp: #116329;
     --prettylights-variable: #953800;
 
+    color-scheme: light dark;
     color: var(--prettylights-fg);
     background-color: var(--prettylights-bg);
     font-family: ui-monospace, SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono",