How to reproduce:
1. Create and open a new project
2. Create a folder in the Project tab that contains the “#“ symbol in it’s name
3. Create a “UI toolkit > Style Sheet“ and a “UI Toolkit > TSS Theme File“ in the new folder in the Project tab
4. Select the TSS Theme File and add a reference to the Style Sheet under the “Style Sheets” section in the Inspector
5. Apply the Changes in the Inspector
6. Observe the Console

Expected results: The Style Sheet is successfully referenced, and nothing is thrown in the Console
Actual results: The Style Sheet reference is not saved, and a warning is thrown in the Console

Reproducible in: 2021.3.44f1, 2022.3.47f1, 6000.0.20f1

Reproduced on: Windows 11 Pro (23H2)
Not reproduced on: No other environment tested

Notes:
- The full warning reads:
“Assets/#UI_Styles/ThemeStyleSHeet.tss (line 13): warning: Invalid asset path: 'Assets/NewStyleSheet.uss' UnityEngine.GUIUtility:ProcessEvent (int,intptr,bool&)“ (could be other paths, names, depending on the names of the folders and files)
- Not reproducible with any other special characters (“$“, “%“, etc.)