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: normal;
+  /* Font weight for interactive elements */
+  --font-weight-element: 500;
+  /* Weight for titles and headings */
+  --font-weight-heading: bold;
   /* 1.5 x font-size for accessibility */
   --default-line-height: 1.5;
   --animation-quick: 100ms;