Using USS Variables

USS Variables are regular USS style properties with custom user-defined names that can hold any style value type. They can then be used in place of a value on any other USS style property. They are primarily used for themes, with the default Unity themes exposing a long list of standard variables to make your UI more consistent with standard controls. Here's a simple example of variable definition and usage in USS:

* {
    --my-var: 20px;
}

.my-button {
    width: var(--my-var);
}

You can assign a variable to a StyleSheet USS style property in UI Builder via the Inspector pane by right-clicking on a style field and selecting Set Variable. This option will temporarily convert the style field into a text field (if it is not already a text field) and allow you to input the name of the USS Variable:

As you type the name of the variable, a dropdown will appear showing any found matching variables coming from either:

1. the current Theme being used for authoring context in the UI Builder via the Theme dropdown in the Viewport toolbar.
2. any assigned StyleSheets to the current UI Document (UXML). The variable dropdown will also let you select a variable option and see its current value and the StyleSheet asset it is coming from. Pressing Enter will set the variable and revert the style field back to its original type.

For style fields that are already text fields, you can also assign a variable by starting to type -- instead of a number.

You can see which style fields already have a variable assigned by looking at the label of the field and seeing if it is incased in a box:

To remove a variable assignment, simply go into the variable mode via Set Variable and delete the variable name.

A few additional points to keep in mind when using USS Variables in UI Builder:

1. USS Variables are not supported in per-element inline styles (in UI Toolkit core). When selecting an element you will still see which style properties are driven by a variable with the field label highlight, but the only option will be to View Variable. You can still override the variable with a real value by overriding the inline style property like normal.
2. At this time, USS Variables are most useful for Editor Extension Authoring, when using the Editor themes (Dark or Light). As such, you will not see a lot of variables being found when searching via the Inspector dropdown by default. Make sure to enable Editor Extension Authoring via the Canvas settings to see all available Editor variables.
3. Currently, UI Builder does not support creation of new USS Variables. To create a new USS Variable you have to do so in a text editor while editing the StyleSheet directly.