feat(theming): Introduce font weight variables

Counterpart of
nextcloud-libraries/nextcloud-vue#8469

Signed-off-by: kramo <git@kramo.page>

Author: kra-mo

apps/theming/css/default.css

@@ -70,6 +70,12 @@
   --font-face: system-ui, -apple-system, 'Segoe UI', Roboto, Oxygen-Sans, Cantarell, Ubuntu, 'Helvetica Neue', 'Noto Sans', 'Liberation Sans', Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol', 'Noto Color Emoji';
   --default-font-size: 15px;
   --font-size-small: 13px;
+  /* Default text font weight */
+  --font-weight-default: 400;
+  /* Font weight for interactive elements */
+  --font-weight-element: 500;
+  /* Weight for titles and headings */
+  --font-weight-heading: 700;
   /* 1.5 x font-size for accessibility */
   --default-line-height: 1.5;
   --animation-quick: 100ms;

apps/theming/lib/Themes/DefaultTheme.php

@@ -194,6 +194,12 @@ public function getCSSVariables(): array {
 			'--font-face' => "system-ui, -apple-system, 'Segoe UI', Roboto, Oxygen-Sans, Cantarell, Ubuntu, 'Helvetica Neue', 'Noto Sans', 'Liberation Sans', Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol', 'Noto Color Emoji'",
 			'--default-font-size' => '15px',
 			'--font-size-small' => '13px',
+			// Default text font weight
+			'--font-weight-default' => '400',
+			// Font weight for interactive elements
+			'--font-weight-element' => '500',
+			// Weight for titles and headings
+			'--font-weight-heading' => '700',
 			// 1.5 * font-size for accessibility
 			'--default-line-height' => '1.5',