Allow themes to specify text color

### Description

Currently, the text color is declared by Glance's CSS file, and from my knowledge, the only way to affect it is changing it's saturation and contrast.

The problem is that there are multiple themes that would use a different text color, for instance, the Catppuccin themes would use their respective `Text` color. This leads to text that doesn't match the scheme the themes are based upon.

For now, the only method to get different text colors would be by using custom CSS, however having to make a CSS file just to change the text color would end up being inconvenient for theme creators.

I would suggest to add an option to disable the default text color, and add the respective text categories.