A good practice to follow is to separate theme specific values in separate .uss files. I found this preferable than using .tss files only because it seems that .tss files do not reload in editor upon saving. The benefit of using .tss files is that you can preview documents in UI Builder under different themes.

For example:


:root {
    --theme-color: white;
    --theme-background: black;


:root {
    --theme-color: black;
    --theme-background: white;


.app {
    color: var(--theme-color);
    background-color: var(--theme-background);

With this you can dynamically add DarkTheme or LightTheme to the XIDocument stylesheets list depending on which theme is selected.


XI Depth

Value (in pixels) that determines how much an element is “pushed” in. To “pop” an element out in front of its parent, assign a negative depth value.

--xi-depth: -4;

XI Z Index

Similar to html/css index. Determines rendering order (only for elements of the same depth).

--xi-z-index: 10;

XI Emissions

Emission values that can be applied to individual colors for emissive glow effect. Value of 0.0 applies no change to color, while a value of 1.0 will multiply the color by 2. Bloom needs to be enabled in project / scene for this to work.

--xi-color-emission: 1.0;
--xi-background-emission: 0.5;
--xi-border-emission: 1.5;
--xi-background-image-emission: 0;

XI Image Rotation and Scale

Rotation and scale values for background images.

--xi-background-image-rotation: 30;
--xi-background-image-scale: 0.5;

XI Font Weight

Similar to html/css font-weight properties. This range from 100 (thinner fonts) to 900 (bolder) fonts. This only works when font-weights are set on the active TMP font asset. See Fonts for more.

--xi-font-weight: 100;

XI Text Decoration

Similar to html/css text-decoration-line. Used to render underline and strike-through / line-through text.

--xi-text-decoration: line-through;
--xi-text-decoration: underline;

XI Font Family

Similar to html/css font-family. Used to set fonts based on names. This property takes a list of names in order of fallback precedence. The first font will be preferred if it exists.

--xi-font-family: 'Noto Sans', 'Liberation Sans';

XI Background

You can find really nice web gradients with the following site:


Similar to html/css background-image. This value can be used to render gradients with an optional rotational parameter and color space paramter.

--xi-background: linear-gradient(red, blue);
--xi-background: linear-gradient(149deg, #00DBDE 0%, #FC00FF 100%);
--xi-background: linear-gradient(149deg, #00DBDE 0%, red 50%, #FC00FF 100%);
--xi-background: linear-gradient(30deg in hsl, red, blue); /* This interpolates in HSL space*/

XI Coloring

This is used to set color targets for different sources. For example, instead of coloring background images with -unity-background-image-tint-color, you can define this property to use the text/font color property instead.

--xi-coloring: background-image color;

XI Cursor Pass

This is used to pass classes from hovered elements to cursor/pointer display. The display must be enabled under XIManager and after you can style like so:

Button {
    --xi-cursor-pass: cursor-hovering-button; /* choose any class name */

.xi-cursor {
    transition: 0.3s border-color, 0.15s width, 0.15s height, 0.15s translate, 0.15s border-width;

/* use the chosen class name here */
.xi-cursor.cursor-hovering-button {
    border-color: rgba(0, 128, 255, 1.0);
    width: 4px;
    height: 4px;
    translate: -2px -2px;
    border-width: 2px;

In the example above, the class name cursor-hovering-button is chosen to be applied to the cursor when it is hovering over a button.

XI Transition

The --xi-transition uss property is a custom property used to animate custom xi properties. By default, Unity UITK does not animate custom properties so this needs to be handled within XI.

Note: The value is a string surronded by quotes although it follows the same syntax as the normal transition property

--xi-transition: "0.3s --xi-depth, 0.3s --xi-color-emission, 0.3s --xi-border-emission";


  • Border radius percentages is not supported (ATM, unable to determine if float value is percentage or pixels with UITK api)

Custom Code-Only / Non-USS Properties

These properties can only be set and modified via code (not available with uss).

  • BackgroundFilter: Texture that applies a masking/filter over the rendered rect background color (or gradient)
  • Visual: A collection of properties used for custom rendering

Pseudo States

Support for pseudo states in XI:

  • :hover - for elements with cursor hovering
  • :active - for active (pushed) elements
  • :focus - for focused elements
  • :disabled - for disabled elements
  • :checked - for checkable elements that are ‘checked’ (toggle, radiobutton)
  • :root - not yet supported in XI1
  • :not - not supported by UITK2
  1. The :root pseudo class applies to top level documents but for some reason this value isn’t being applied in UITK when set in XI. 

  2. Because UITK does not support the :not pseudostate in USS, it cannot be used in XI. This state is commonly used in HTML/CSS web applications and is useful for styling the negation of other classes or pseudo classes. 

XI - Copyright © Jonathan Lacombe