refactor(ui): standardize the border radius (#1920)

Impact:
- Increase the border radius of inline code and `<kbd>` tags
- Variable `$base-radius` renamed to `$radius-lg`

_sass/addon/syntax.scss

@@ -102,9 +102,9 @@ code {
 
   &.highlighter-rouge {
     font-size: $code-font-size;
-    padding: 3px 6px;
+    padding: 3px 5px;
     word-break: break-word;
-    border-radius: 4px;
+    border-radius: $radius-sm;
     background-color: var(--inline-code-bg);
   }
 
@@ -261,7 +261,7 @@ div {
     .content > & {
       @include ml-mr(0);
 
-      border-radius: $base-radius;
+      border-radius: $radius-lg;
     }
 
     .code-header {