The html document type doesn't do CSS highlighting inside <style> elements - it should work the same way that JS code is highlighted properly inside <script> elements.
Styling is performed in the Scintilla editing widget Geany uses. This is a separate project and you need to submit the feature request [there](https://sourceforge.net/p/scintilla/feature-requests/) if it does not already exist.
It is an open issue. but has been open for more than 10 years! (https://sourceforge.net/p/scintilla/feature-requests/470/) I've been able to tolerate this problem because I love Geany, but now I'm spending a lot of time working on single file VueJS components which have html, css and js in one file so it's become a big problem and I'll need to find a different editor :-(
Unfortunately nobody who needs it is likely to have the C++ experience to be able to implement it, and as Neil noted back in 2009 it is a complex job, so somebody who doesn't need it is unlikely to do it.
I am afraid thats the reality of open source that is not company supported.
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