All we need is an easy explanation of the problem, so here it is.
How to set some other theme in incognito mode? Currently bar is so dark, I can’t clearly see anything. This is what I used to use:
- In experimental (chrome://flags/) I enabled "Allow widgets to inherit native theme from its parent widget" function.
- I installed plain theme.
Starting today, it stopped working. After every problematic update I always managed to find some workaround.
How to solve :
I know you bored from this bug, So we are here to help you! Take a deep breath and look at the explanation of your problem. We have many solutions to this problem, But we recommend you to use the first method because it is tested & true method that will 100% work for you.
Method 1
For me custom themes also stopped working in incognito mode today.
They already stopped working some weeks/months ago and I was able to fix it with the #inherit-native-theme-from-parent-widget flag set to "Enabled"
I was able to get them working again by setting the flag chrome://flags/#incognito-brand-consistency-for-desktop to "Disabled". After that I was able to return chrome://flags/#inherit-native-theme-from-parent-widget to "Default" and themes seem to work again in Incognito mode.
Method 2
In chrome://flags search for
"Enable Incognito brand consistency in desktop."
Set this flag to disabled. Then the incognito theme is what you defined for non incognito mode.
Note: Use and implement method 1 because this method fully tested our system.
Thank you 
All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0