Branding > The CSS is still applied if a theme is disabled

I've a Branding1 as my default theme and no CSS in it.
I've a Branding2 with a CSS with content - visually disabled - since it's not my default theme.

Even if from the dev side the CSS is not linked to one branding, it's not understandable from the user side, and can be really hard to debug from the Studio side if a style is not working or in conflict with default styles.

Is there a way to link a branding with a CSS ?