Proton defines its colours in theme manifests and browser-custom-colors.inc.css. In some situations, neither is read: for example, when the system theme is applied on Linux or in High Contrast Mode. To handle this, we've defined fallbacks for those colours. Most of them are in browser.inc.css. However, some fallbacks are defined at the point the rule is used, like is the case for buttons-primary-button-* rules. We should settle on one approach. My preference is for the former, so fallbacks are consistent everywhere.