One improvement could be a better way to group and elide units, e.g.

Instead of

.border_1().border_color(rgb(0x0000ff))

You get

.border(1 0x0000ff)

(guess, this can only be in a macro)

Or at least have "color" which understands that it's allied to border instead of having to repeat "border"

> I don't think that it can be substantially better

Don't give up too early, there is no way it can't be, we don't really have any good systems optimized for ergonomics to reach anything great

> .border(1 0x0000ff)

After 10 or so of those you'll start dreaming of

   border-left: 2px solid rgb(0 0 100%);