Since styles_for_scope effectively ignore the font parameters (and instead return styles based on the font parameters used when the cache entry was created) it is a better design to have theme_t own the font. This way, changing it can do the proper cache invalidation.
A minor downside is that we can now change the font of the theme without also changing the layout’s font, which wouldn’t cause proper layout invalidation — since layout_t owns theme_t though, users of a layout should always change font via layout_t (which will then change it in the theme).
Also, themes cannot be shared between views that wish to use different font settings.
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