[oliverrealize]

#buddypress form#whats-new-form #aw-whats-new-submit {
    background: transparent;
}

The above is what chooses the color of the button. Just replace transparent with a hex. i.e. #000000 for black.

You can also limit it to just the activity page with the activity pages class which is .activity:

.activity #buddypress form#whats-new-form #aw-whats-new-submit {
    background: transparent;
}

I’d be careful with the .activity class, it’s a really vague class so other plugins, themes may use it.