Allow controlling the lightness of theme colors

[apcamargo]

As mentioned in #9618 (comment), the only way to change the lightness of theme colors is to first choose a preset with the desired lightness and then adjust the hue and saturation using the circles and the slider.

It would be helpful to have a separate slider to control lightness directly. Right now, it is not clear what the current slider controls, although it seems to affect saturation.