*Steps to reproduce:*
# Create a new URP project.
# Assets > Create > UI Toolkit > UI Document.
# Open this in UI Builder. Under StyleSheets, press + > Create new USS.
# Add multiple things to the Hierarchy, make changes, but don't save.
# *User thought:* "Oh, I'd like to create a variable to use with my UI. Let's open (the USS made in Step 3) and add one."
** Open the USS in an IDE/text editor and add:

{code:java}
:root {
--color-1: blue;
--color-2: yellow;
}{code}
To the USS file you created in Step 3. Save your file.

6. *User thought:* "Let's put this variable into the UI Builder"
*
** Return to the editor.

*Actual results:* A warning pops up that the .uxml was changed outside of the UI Builder with the following description.
{code:java}
The UI Builder will now apply the changes made outside of the UI Builder. This overwrites any unsaved changes you made in the UI Builder.
Note: To avoid conflicting changes, make sure to save any changes you make in the UI Builder before editing a file in an external editor or in a dedicated editor inside Unity (this includes resource files like Font and Sprite assets which have their own editor).
{code}
* *(!)* The user can only press OK, or X, _+*both of which lead to all unsaved changes being wiped away.*+_

*Expected results:*

As the user, I only wanted to add a USS variable, or edit the USS variable to slightly tweak the UI. If this had to happen, I would have *much* rathered:
* An option to save UI Builder changes first.
* An option to undo my USS change, which when discovering this issue was just me editing a value from 36px -> 28px. This would have been a much less painful loss of data.
* A choice on which to keep.

Being told there is no option but to lose the UXML changes while those changes are still visible in the background was quite sad.

*Reproducible with versions:*
* 6000.1.0b2, 6000.0.35f1, 2022.3.56f1, 2022.2.0a17

*Tested on (OS):* Windows 10

*Notes:*
* Editing the USS seems to be the only way to add variables at the moment. The documentation tells you that you need to add them in a text editor: [https://docs.unity3d.com/6000.0/Documentation/Manual/UIB-styling-ui-using-uss-variables.html]