Add a banner to the default theme?

There is a lot info about themes but they all suggest to make a new theme .
I only want to replace the DEMOKEYCLOAK on the picture with one Logo i have
Can i edit an existing theme for such a small change? I have make a new realm but everything is in default.

That gets built with the “HTML Display name” realm attribute. But, because Keycloak sanitizes that string, it won’t allow you to include external images. In order to get a logo there, you’ll have to build a custom theme.

I have a theme starter here: GitHub - xgp/keycloak-theme-template: Starter for building a Keycloak theme