Fix border-widths that crash Vite when compiling

[Floppy]

I've been using sassc to build my CSS for ages, and bootswatch has been working great. However, I changed to Vite recently, and it would crash without any extra info when compiling the Vapor theme. I narrowed it down to this $border-width variable - if it's set to 0, it crashes, if it's 0px it works fine.

I don't know why this is the case either on the Vite or the Bootswatch side, but it does seem to fix it. I wondered if some calculation is done elsewhere inside Bootstrap using border-width that gets confused if the px specifier isn't included, perhaps.

[Floppy]

OK, I found out where it's going wrong; I'm also including tom-select, which has this in it:

min-height: calc(1.5em + 0.5rem + calc(var(--bs-border-width) * 2));

So, yeah, I guess Vite is crashing because of that calculation and the fact that the border-width as set is dimensionless.

[Floppy]

BTW, I appreciate that this might well be the wrong place to fix my problem. I'm trying to create a reproduction for Vite as it's probably their bug at heart, but haven't got there yet. This PR is open in case it's a useful change for you anyway.