Feature request: add css custom properties to simplify downstream styling

[magicDGS]

Some specific styling downstream might require using pixel sizes that depends on draw configuration and then hardcoded at the attribute (i.e., on rect, the width and position attributes), or there are hardcodes styles that are commonly overwritten when custom styling (i.e., font-* styles on different locations or colors).

Using custom properties would help in overriding some of them on specific keys to simplify the process, by overriding them on some selectors. In addition, it can simplify the process of customization and split it on two configuration ways (that can be combined):

• Add a configuration entry for custom-properties override, that will just enable to use different fonts, colors, sizes, etc. and enable some basic configuration without advance css.
• Using the current configuration for extra styles, would enable to use calculations and override of custom properties for more advance styling