Checking text display quality by software for XFCE

Today I have switched the colour theme setting (example: Breeze) for my XFCE session.
I got the impression then that some text is not displayed so well as I became used to. Extra colours are displayed besides vertical black character lines at places like on buttons from the task bar window list or output by the program “xfce4-terminal”.

I find the display quality still better by the program “konsole 18.12.0-1.1” at the moment.

Should I try any further system fine-tuning or another software update out?