Add border radius SCSS variable

The following discussion from !151926 (merged) should be addressed:

• @thutterer started a discussion: (+1 comment)

  suggestion (non-blocking): I think we repeat $border-radius-default - 1px often enough to make it a SCSS variable, scoped to this block 🤓

  That would make the this block much easier to read and reason about. Something like $inner-border-radius: $border-radius-default - 1px; would do.