Revert "Move button and caret colors to a variable"

This reverts commit 4c7e1e2b905ae31683574ec12c6fc9942cf9f611.
Revert "Move button and caret colors to a variable"

This reverts commit f6ab8294bffbbf1e881ee2ec99ec4ca41e068599.
Move button color and caret border to variables.less

This ensures that button text and dropdown carets will
be the same color.