To add a StyleSheet to the current UI Document (UXML), use the + dropdown menu and select one of the following options:
You can also right-click anywhere inside the StyleSheets pane tree view to access these option menu.
To remove a StyleSheet from the active UI Document, right-click on the StyleSheet and select Remove USS. This requires a full save of all unsaved changes.
A single Active StyleSheet always appears bold. If you create a new USS Selector from the Create new selector… field, it’s added to the Active StyleSheet. To change the Active StyleSheet, right-click on a StyleSheet and select Set as Active USS.
When editing Sub-Documents, the StyleSheets that belong to parent documents appear grayed out. These provide context but are otherwise read-only.