Add a way to install sourceview style schemes
You can add more color schemes to builder by copying the xml files to the GtkSourceView folder, but flatpak and sandboxing have made this a bit more complicated (is there a common folder for these anymore?).
It would be better if there was a way to install themes from the builder preferences. The color schemes list could become an editable list. The Add button would ask for a xml file and copy it to the right place.