Add some color to the toolbar buttons (Add Files, Start Encoding, etc.)

[unittwentyfive]

I'd like to suggest having the option to show the button colors in the toolbar at all times, not just when the mouse is over the button. As someone with limited vision, having the greyscale buttons just makes everything harder to find Even if there were a duller version of the colors that lit up brighter when the mouse went over them, that would still be great. Making it selectable between the current greyscale and full-color (and even that third dull/full color mode), would allow those that like the dark theme to keep it dark, and those of us who could use the visual aid to have the option.

Thank you!