feat: remove dark mode toggle, switch to prefers-color-scheme

This means it's not possible to manually change the theme, but all major operating systems
support global dark mode now, so it shouldn't be a huge problem.
Will re-add the dark mode toggle if the Sec-CH-Prefers-Color-Scheme header gets added to Firefox and Safari.

Foxnouns.Frontend/app/app.scss

@@ -1,20 +1,7 @@
-$font-family-sans-serif:
-	"FiraGO",
-	system-ui,
-	-apple-system,
-	"Segoe UI",
-	Roboto,
-	"Helvetica Neue",
-	"Noto Sans",
-	"Liberation Sans",
-	Arial,
-	sans-serif,
-	"Apple Color Emoji",
-	"Segoe UI Emoji",
-	"Segoe UI Symbol",
-	"Noto Color Emoji" !default;
-
-@import "bootstrap/scss/bootstrap";
+@use "bootstrap/scss/bootstrap" with (
+	$color-mode-type: media-query,
+	$font-family-sans-serif: ("FiraGO", system-ui, -apple-system, "Segoe UI", Roboto, "Helvetica Neue", "Noto Sans", "Liberation Sans", Arial, sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol", "Noto Color Emoji"),
+);
 
 @import "@fontsource/firago/400.css";
 @import "@fontsource/firago/400-italic.css";