Use a shared CSS variable for border colors

[agex-n7]

While customising the design, I noticed that components reimplement the border style; in particular, they redefine which colours to use for every different component (like the card).
So my suggestion would be to introduce a common CSS variable for this, e.g. --mantine-color-border, and use it similarly to --mantine-color-text.

I realise that --mantine-color-default-border exists, but from what I can tell, it is only applied to default variants.

If there's interest in this change, then I can introduce the PR for this.

[rtivital]

I'd rather not do that for now. It will most likely not work in all cases as border color may change depending on component.