Unfortunately the design tokens are only available in MW as Less variables at the moment, so they can only be used in Less styles. On-wiki styles (Gadgets, user styles, TemplateStyles) can only use CSS, not Less. I looked into fixing that, but unfortunately the lack of Less support was a deliberate decision. So instead, we need to find a way to provide the design tokens in CSS. I filed this task for that a few months ago, as part of a larger effort to support Codex in Gadgets / user scripts, which we plan to work on in the coming months.
So in summary, tokens are not currently available in on-wiki styles due to technical barriers, and we plan to fix that some time in the next few months. Once that is fixed, I completely agree that using the tokens for on-wiki templates/modules would be a great idea. In addition to making the color scheme more consistent, using tokens would probably also help make those things compatible with the upcoming dark mode feature. (Some of the technical architecture decisions around dark mode are still unsettled and I'm not in charge of them, so I can't make any promises. But I think that "encourage on-wiki content to use CSS variables that correspond to (a subset of) Codex tokens" or something similar to that is likely to be part of the plan.)