I changed SAP HANA studio to the dark theme, but the text in the editor is difficult to see. I've searched through all the preferences for about 30 minutes but I still can't find any way to change this.
There are some posts in the SAP Community asking this same question (with no solution yet):
https://answers.sap.com/questions/12980770/dark-theme-with-hana-studio-not-supporting-sql-scr.html
https://answers.sap.com/questions/12895423/how-do-i-change-the-font-color-in-hana-studio-sql.html
SAP HANA Studio
Version: 2.3.41

The build-in options to change the color themes are pretty limited.
Given that SAP does not further develop SAP HANA Studio - it is deprecated and superseded by the WebIDE - the only option I found is to use a 3rd party tool to change the theme:
Darkest Dark Theme with DevStyle from Genuitec.
I just added the "software site" URL (
http://www.genuitec.com/updates/devstyle/ci/) to the HANA Studio Eclipse and installed the DevStyle product.This resulted in a look like this:
It does not provide detailed control over the syntax-highlighting, though. So if you're after choosing which SQL keywords get which colour then you're out of luck with SAP HANA Studio.