I would really like to contribute a PR to change the colors it has been on my mind since I created the Fast Templates.
Would you create a Feature Request on Github? Try to describe your use case and requirements.
The Fast Template is built on the https://fast.design framework. If you take a look at the site you will realize that style is set by a special fast html element - not the css. I.e. you can tweak the look and feel by changing only the background, base and accent color.
There is a quite impressive tool on the site where you dynamically can change the look of their site via a few sliders.
Speaking about changing the colors in these FastTemplates (which are really great), there is one slight problem which concerns the background color of the area where options are shown in MultiChoice widgets. It is completely transparent which yields readability issues, as it is superimposed to the content above and below the widget.
I have looked into the css and it corresponds to background-color in .bk-root .choices__list--dropdown in fast_bokeh.css. I was able to change it—for instance setting it to the actual background color of this theme, i.e.#F7F7F7—in place from within the navigator. However, in my case changing the local file does not change anything when I reload my app.
Also, in contrast to the native theme, the item above which the mouse hovers is not highlighted, but that’s a more minor issue.