ButtonGroup and ButtonMenu Should Pass "theme" Prop Down

[machineghost]

ButtonGroup and ButtonMenu components can already pass down a size property to their buttons, so:

<ButtonGroup size="sm">
  <Button>Foo</Button>
  <Button>Bar</Button>
</ButtonGroup>

is the same as:

<ButtonGroup>
  <Button size="sm">Foo</Button>
  <Button size="sm">Bar</Button>
</ButtonGroup>

In the same fashion, it would be nice if you could set "theme" on a group/menu, so that (for instance) if you wanted a group of dark buttons you could do so with:

<ButtonGroup theme="dark">
  <Button>Foo</Button>
  <Button>Bar</Button>
</ButtonGroup>

This seems like it would be simple enough to add, so I could potentially submit a PR if desired.