When I try to use variables (e.g. var(--text-color)) that are defined in my Common.css inside a /style.css for my Template for font colors, font families, etc., I get an error saying "Invalid or unsupported value for property font-family at line 12 character 18." Any solution for this?
Topic on Help talk:TemplateStyles
Not yet, short of inlining the definition in the TemplateStyles page or not using TemplateStyles. See phab:T320322.
Seems like Extension:TemplateStylesExtender (linked from the Extension:TemplateStyles) would be the solution, if the only problem is that the basic TemplateStyles doesn't support CSS variables.