I am not a Thea user but I doubt whether even an experienced one can see what is wrong from a single screenshot. “Too much light” or “overexposure” would be my guesses.
Maybe you need to share the file on the Thea forum. The settings you’ve made for the Thea scene should be available for others using Thea to review. Or you can post the file here.
First look at “3d Lut” and “Filtering” if you have settings there. I 'd leave those settings alone / off until you’ve resolved the primary settings under “Exposure”
Since this looks like natural lighting. I’d say as a guess: Try setting the Exposure under “Display Settings” to “Filmic” and in the Hamburger menu choose “Interior Sunlight”.
Since it looks like the problem is not due to artificial lighting, maybe Exposure js the issue, Choosing a setting in the Exposue hamburger menu will reset the f-number, ISO, and Shutter to normal levels.
There is one “artificial lighting” effect to consider. It could be you have Ambient Occlusion settings under “Presto (MC) Settings” checked ON. Try unchecking that. then, after you get Exposure under control, work with the Ambient Occlusion (SO) settings to get what you want. You may have the settings too high, and some experimentation with AO AFTER you’ve knocked out the over-exposure otherwise will work for you.