Feat: Add temporary border-radius variable

Signed-off-by: Marco Ambrosini <marcoambrosini@proton.me>

apps/theming/css/default.css

@@ -49,6 +49,7 @@
   --default-font-size: 15px;
   --animation-quick: 100ms;
   --animation-slow: 300ms;
+  --border-radius-element: 10px; /** Nextcloud 29+ | temp variable to use old design */
   --border-radius: 3px;
   --border-radius-large: 10px;
   --border-radius-rounded: 28px;

apps/theming/lib/Themes/DefaultTheme.php

@@ -183,6 +183,7 @@ class DefaultTheme implements ITheme {
 			'--border-radius' => '3px',
 			'--border-radius-large' => '10px',
 			'--border-radius-rounded' => '28px',
+			'--border-radius-element' => '10px',
 			// pill-style button, value is large so big buttons also have correct roundness
 			'--border-radius-pill' => '100px',