As it happens, there is a new (undocumented) way of accessing these colors in a template since last week when the themes were rewritten by @kubawolanin:
The syntax below:
{{ variable | themeValue:'primary-color' }}
(leave out the {{ }} if you’re already in an expression, obviously)
will give you the “primary color” of the current theme (for example for the Material theme it will be red) if variable is not set. This is useful so you can replace variable with e.g. a color config option of your custom widget (config.your_color_setting for instance) to provide a fallback if the setting is not set. Similarly, if you want it to always return the theme value simply replace variable by null.
See here for a list of the theme variables (ignore the -- prefix):
Examples are box-bg or switch-off-color.