The tt is a shortcut for “toggle-theme”. It is more of a dev easter egg which can be confusing to use (like me and others) if you don’t know what you accidentally pressed and the theme got changed permanently 
The keyboard shortcuts in v.9.3.6 are “ct” and “cr”.
I’ll re-open this on github because this issue took me 15 minutes to figure out, and I’m sure many other users will still get annoyed by it.
This theme switch was triggered for me when typing a search string into a variable entry box, so it’s not correct behavior at all.
I’ll request developer shortcut keys are disabled by default.
Im having the same issue but the fixes dont quite work for me, when I do “ct” it goes back to dark mode and I see a pop up saying “Preferences Saved”, but when I refresh its back to all white…
Hi @tzeltzer,
Welcome to the 
community support forums !!
We are excited that you joined our OSS community. Please read about some of the FAQs in the community 
Please see the above GitHub link as more details.
If incase you cannot find a solution then feel free to ask in the GitHub comment.
Hi. I got the same and solved it by clicking on my profil icon on the top right of any screen.
then select "Profile’, and select ‘Dark’ on Interface Theme.
Ctrl+k and type “theme” and later change the theme to dark 
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Just change language to English or other, and change back
worked for me! @pereus
There is some key stroke that either toggles dark/light mode or just switches to light mode that I obviously keep accidentally hitting, then its a right pain to get back into dark mode - I literally NEVER want light mode and wish I could lock it into dark mode.