RibbonControl ships its visual resources in avares://RibbonControl.Core/Themes/Generic.axaml.
Recommended application setup:
<Application.Styles>
<FluentTheme />
<StyleInclude Source="avares://RibbonControl.Core/Themes/Generic.axaml" />
</Application.Styles>
Resource guidance:
xmlns:ribbon="https://github.com/wieslawsoltes/ribboncontrol",FluentIconData,The sample apps also show how to override RibbonIconPresenter themes with a ControlTheme scoped to the application resources.