Purple tor buttons use white text in both light and dark theme

Current both light and dark themes use #ffffff white text for the purple tor buttons, like ".onion available". We should use an almost-white text in the light theme, and almost-black text color for the dark theme.

From #41608 (comment 2895231):

in the figma mockups the "Connect" button uses almost-white text with a light theme, and almost-black text with a dark theme.

In contrast the ".onion available" urlbar button always uses white text, regardless of theme:

And the reply from @donuts

The two variants are supposed to use purple-60 (light theme) and purple-30 (dark theme) for backgrounds, and the normal label color for foregrounds (e.g. page-color, I think?).

For consistency, maybe we should ignore the dark theme variant for now, and we can open a separate issue to fix this everywhere (i.e. torconnect, onion-location, and this instance).