CSS syntax that civilint can't handle

Trying to pass the civicrm-core styles has thrown up a couple of bits of CSS it doesn't understand:

:&

I also don't understand this, isn't this SCSS? I've tried to break it out.

• https://lab.civicrm.org/extensions/riverlea/-/blob/main/streams/thames/css/civicrm.css?ref_type=heads#L363
• https://lab.civicrm.org/extensions/riverlea/-/blob/main/streams/thames/css/civicrm.css?ref_type=heads#L549

:is(.class-a, .class-b)

The linter forces a line break on these, which to me garbles the style, but I think doesn't effect the css functionally?

Hopefully we can update this rule, but I've given in to it in the short term.

I've attempted to fix with https://github.com/civicrm/civicrm-core/pull/31478/commits/d0422f3ac85fea158fbc4171c64aaf38cbb02121 but would you be able to have a look @nicol @artfulrobot ?