Hi - today I installed another Windows 10 and then apart from the other software I installed the latest version Calibre 7.17.0.
As recommended when you want to use the configuration you made in your previous installation, which was the version 5.24.0, I replaced the files in the Roaming Calibre folder (C:\Users\user\AppData\Roaming\calibre) with the old ones. The result was exactly as described here - main Calibre screen look washed-out and with thin fonts...
I can see that Mr. Kovid reaction here concerning this problem is similar as it was when people asked for his help with the "new" Calibre e-book viewer which after the update was taking excessive time to open - he was unable to tell us what to do, how to fix it...
I uninstalled this new Calibre version and tried some older - no improvement, until I have found that the version 5.44.0 works OK. Everything above starting with the version 6.0.0 seemed to work OK with my configurations, but the main Calibre screen always look washed-out with thin fonts.
At least the Calibre e-book viewer works OK.
I tried those recommendations concerning changing the Calibre.exe propertis, but they did not help.
When testing the later versions I have found that the wash-out screen was related to my old config file called gui.json. Once I inserted it to the Calibre folder (C:\Users\user\AppData\Roaming\calibre) the screen changed showing my configs as I wanted, which was OK, but at the same time the screen became washed-out with thin fonts... The problem with the screen happens since the version 6.0.0 and further.