[5.x] Make active toolbar buttons of Bard more visible in dark mode

[carstenjaksch]

Problem:

In dark mode, the active toolbar buttons of Bard are barely visible:

Compare that to light mode:

Solution:

White text color for a nice "glowing" effect and less darker shade (600 instead of 800) for active buttons. For hover, I reduced that from 700 to 650 for a little more visibility, but not too much to steal the active button's show!