Can I change RadzenMenu background-color porperty somehow?
I have tried in site.css and inline in Style property but nothing happend.
Yes, you can change the menu background color via Radzen's theme customization. You can also override the theme CSS - use your browser's developer tools to inspect the rules that define the background color.
I have tried to override it in the site.css, as I mentioned, but default-base.css re-override it despite of !important rule.
I am going to see the theme customization.