remove hard-coded theming

[cezaraugusto]

theming is made via props by theme provider and should not be hard-coded.

from @petemill in https://github.com/brave/brave-ui/pull/134#pullrequestreview-153521708

To support themeing correctly, we shouldn't call 1 specific theme directly in the style files, but rely on the root <ThemeProvider and simply use props.theme.x which are provided by the ThemeProvider. See the <Button component as an example of how to define Prop types with typescript for styled-components to automatically get the compiler OK for these props, and good intellisense.

I agree