Is there a way to add *.uss for Unity stylesheets and then apply the custom rules? Follow

Hey all,

    I was checking out custom variables in Unity StyleSheets (USS), and Rider kept saying that it was invalid in the places below, but I took the info right from Unitys documentation: https://docs.unity3d.com/2020.3/Documentation/Manual/UIE-USS-CustomProperties.html

:root {
    --header-background: rgba(255, 23, 0, 0.29); // -- Gives error "Unknown pseudo selector 'root'"
}

.controlsLabelRowContainer, .controlsLabelIORowContainer {
    background-color: var(--header-background); // -- Gives error "Cannot resolve '--header-background' custom property"
}

Granted, when I saved and refreshed Unity, the variables still worked, but it would be nice to be able to have Rider in sync with the actual USS rules/inspections. Is there a way for me to add USS? It looks like it keeps trying to use CSS and I have been having an issue trying to figure out how to add a new one.

Thanks,

Dan