with theme.css (correct way) or css injection (easy way)
for something like this injeciton seems good.
wrap it in reverse brackets }{ and put it into a styleclass backgroundimage.
no need to use the styleclass anywhere, this will get it injected into the css file xd
the correct way would be to create your own css file…
a theme must be created here C:\Program Files\Inductive Automation\Ignition\data\modules\com.inductiveautomation.perspective\themes
and import your default theme file (light css) and set your default theme to the one you just made.
Do not edit existing themes as these will be lost on an update
Btw its possible that this also affects alarms notification. so test that out