fix #15182

Description

It's a bug relative to 2f98a35

UI is looking for http://localhost:7863/file=/app/webui-assets/css/sourcesanspro.css but the assets repo is mounted at http://localhost:7863/webui-assets. It is because style.css is loaded at http://localhost:7863/file=/app/style.css and @import url() with relative path.

Screenshots/videos:

Checklist:

• I have read contributing wiki page
• I have performed a self-review of my own code
• My code follows the style guidelines
• My code passes tests