Fix some button border colors in dark mode

What does this MR do?

Get some borders colors in dark mode to cool it a little.

1. Darker value for borders that use $border-white-normal in dark mode - use #4f4f4f instead of #f0f0f0
2. Don't bother setting $border-white-light, it is no longer used since (new emoji menu doesn't have .emoji-menu class, which is the only usage)

before	after