In UI Toolkit, you can use USS to customize the appearance of visual elementsA node of a visual tree that instantiates or derives from the C# VisualElement class. You can style the look, define the behaviour, and display it on screen as part of the UI. More info
See in Glossary. The suggested workflow for USS is that you visually style an element in UI Builder, extract the style to a USS file, and then reference it in UXML.
If you style an element in UI Builder, the style is added as an inline style to the style attribute of UXML elements:
<ui:UXML ...>
<ui:VisualElement style="width: 200px; height: 200px; background-color: red;" />
</ui:UXML>
To reference a stylesheet file, add the <Style> element under the root element of a UXML file.
For example, if you have a USS file named styles.uss in the same folder of the UXML file with the following content:
# root {
width: 200px;
height: 200px;
background-color: red;
}
You can reference it like this:
<ui:UXML ...>
<Style src="styles.uss" />
<ui:VisualElement name="root" />
</ui:UXML>