Implement support for dedicated theme CSS files from our gresource.

Properly implement light and gruvbox themes. Yay!

theme/meson.build

@@ -1,12 +1,22 @@
 sassc = find_program('sassc', required: true)
 
-theme = custom_target('Theme generation',
-	input: 'main.scss',
-	output: 'style.css',
-	command: [
-		sassc,
-		[ '-a', '-M', '-t', 'compact' ],
-		'@INPUT@', '@OUTPUT@',
-	],
-	build_by_default: true,
-)
+builtin_variants = [
+	'dark',
+	'gruvbox',
+	'light'
+]
+
+themes = []
+
+foreach variant: builtin_variants
+	themes += custom_target('@0@ theme generation'.format(variant),
+		input: 'koto-builtin-@0@.scss'.format(variant),
+		output: 'koto-builtin-@0@.css'.format(variant),
+		command: [
+			sassc,
+			[ '-a', '-M', '-t', 'compact' ],
+			'@INPUT@', '@OUTPUT@',
+		],
+		build_by_default: true,
+	)
+endforeach