fix: fix occasionally invisible box-shadow in Edge

What does this MR do?

Use px for border widths rather than rem. We don't need border widths to scale based on user's font size

with px-to-rem we were getting rounding issues:

    px-to-rem(1px) == 0.06rem
    0.06 * 16 = 0.96, i.e. less than 1px!

Does this MR meet the acceptance criteria?

Conformity

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