Why doesn't my UI Toolkit USS apply at runtime?

PanelSettings or UIDocument's Theme Style Sheet is empty, or the USS isn't referenced from the UXML. Add stylesheet via inspector reference or styleSheets.Add at runtime.

PanelSettings.themeStyleSheet for global vars (font, colors). Per-document USS for component-specific styles.

Save USS file in editor; UI Toolkit auto-applies in PIE. In standalone player no hot-reload — rebuild required.

Fix: UI Toolkit USS Not Loaded At Runtime

Quick answer: UIDocument inspector → assign Source Asset (UXML referencing USS), or programmatically root.styleSheets.Add(uss).

Style file looks great in builder. In runtime build, none of the styles apply. UXML didn't reference the USS, or load order broke.

The Fix

/* myView.uxml */
<ui:UXML>
    <Style src="myStyles.uss" />
    <ui:VisualElement class="container" />
</ui:UXML>

// Or runtime
[SerializeField] StyleSheet _stylesheet;
void Start() {
    var root = GetComponent<UIDocument>().rootVisualElement;
    root.styleSheets.Add(_stylesheet);
}

Style src in UXML is the declarative path. styleSheets.Add for dynamic theming.

Verifying

Styles apply in build. Without reference: default flex layout, no custom styling.

“Reference USS. Styles apply.”

Related Issues

For UXML binding, see binding. For flex-wrap, see flex-wrap.

Reference USS. Styles flow.