fix: improve dark mode styling and contrast

- Set dark mode as default (class="dark" on html)
- Update dark mode CSS variables for better contrast
- Fix popover/select dropdown colors
- Improve SelectItem hover and focus states

index.html

@@ -1,5 +1,5 @@
 <!doctype html>
-<html lang="en">
+<html lang="en" class="dark">
   <head>
     <meta charset="UTF-8" />
     <link rel="icon" type="image/svg+xml" href="/vite.svg" />