Dashboard theme reseting to default randomly

HI, although I have set some preferred colors to the dashboard theme, it resets to default white/blue some time to time, would you know why ?

I did observe the same, indifferent pattern only when I heavily customize my nodes. As soon as I leave everything as it is, it runs stable.

My workaround is to accept those resets and to re-use my saved theme settings every time it occurs.

The Light is set as default... how do we set the dark as default ?