Calibre is just a qwidgets based pyqt program. Since its as KDE project the "modern" UI stack would be Kirigami QML and you can use QWidget::createWindowContainer to embed QtQuick parts in a QWidgets program. With signals and slots they can still communicate, so you can peacemeal replace parts of the UI like that, but looking at the sources the main window is composed of about 30 different parts between a half dozen folders so thats pretty immutable.
Immutable is the right word. I was trying to “fix” the new dark mode in Calibre 4 (it’s bad and clearly not tested — the default hyperlink colors are nearly unreadable and the hover-over color in the library menu are impossible to read), and while I could fix the CSS in some parts (which was the dev’s dismissive answer to the bug report on how dark mode is legitimately broken), other parts can’t be fixed without using overly-complex plugins (that have to apply overrides every time you start the app) or ripping apart the source in a way that is not worth the effort.
My solution was to use a different Mac app to “force” Calibre to ignore the systemwide setting and use its light mode instead.
