Ignition 8: root container background color changes unexpectedly

I’m working at a project on Ignition 8.1.3 (b2021030309) (but the same issue occured in 8.0.16 too).
My root container background is set to 250,250,251 as default value. Sometimes all of a sudden the background color changes to 130,138,140 unexpectedly, just while editing the project in the designer.

Any idea?

Thanks, regards