Default icon themes from plugins are not properly applied the first time

[martin-fleck-at]

Bug Description:

Default icon themes from plugins are not properly applied the first time since they are registered after the preferences have been initialized. Once they are set and you reload/restart the application, everything works fine since the theme is stored in the local storage and applied as soon as it is registered. However, if there is no prior theme set, the icon theme service believes that the theme is already set correctly since the default value was also changed.

Steps to Reproduce:

0. Setup available on this branch as well: https://github.com/martin-fleck-at/theia/tree/issue-10937-case
1. Install any icon theme from vsx into your product, e.g., https://open-vsx.org/extension/PKief/material-icon-theme
2. Set the default icon theme for your application to that theme, e.g., "defaultIconTheme": "material-icon-theme"
3. Start up the (browser or electron) application and see if the theme is applied on a clean state. You can get to a clean state by clearing the browser data or removing the respective electron directories.

file-icon-theme-application.mp4

Additional Information

• Operating System: Ubuntu but probably all are affected.
• Theia Version: current master