Ok never mind, I have solved my problem by duplicating the theme and reducing the transparency to 0.
It works well for now.
I bet this problem come from the lack of support of Nvidia graphic cards in MacOS Catalina...
I heard that Apple and Nvidia aren't in good terms now, so apple is not using Nvidia graphic cards anymore and Nvidia doesn't update the drivers...
And off course, the one who endure the consequences is the poor customer that have a 2014 macbook pro
Thanks anyway