Side buttons from Distribution Wizard are not displayed properly anymore

Steps to reproduce

The issue is visible especially when upgrading to XWiki 18.4.0.

1. Create an older instance of XWiki with a subwiki (e.g. I've tested and reproduced with XWiki 12.10.11 Jetty/HSQL)
2. Copy the data folder from this older instance into XWiki 18.4.0 instance folder (replace the existing one if exists) (optional, I also deleted the tmp folder)
3. Start the XWiki 18.4.0 instance and access it in the browser
4. Follow the Distribution Wizard until the upgrade is finished on Main Wiki as well as on subwiki
5. Observe the side-buttons (e.g. on Later, Upgrade, Continue, etc)

Expected results

The side-buttons are displayed normally next to the right side of main buttons, having a similar shape.

Actual results

The side-buttons are displayed in the right side of the main buttons, but are very small and not having an icon.

Some of the affected side-buttons are:

On Main Wiki DW

• Later side-button, on the first screen of DW
  
• Upgrade side-button, on Step 2 - Flavor
  
• Continue side-button, on Step 2 - Flavor
  
• Uninstall side-button, on Step 2 - Flavor
  
• Check for updates and Upgrade side-buttons from Step 4 - Extensions
  

On subwiki DW:

• Later side-button, on the first screen of subwiki's DW
  
• Continue side-button, on Step 1 - Flavor
  
• Uninstall side-button, on Step 1 - Flavor
  

The changed shape of the side button can be observed when just creating an instance of XWiki 18.4.0:

but the bigger problem can be observed when upgrading.

The issue could not be reproduced on XWiki 18.3.0 or XWiki 17.10.9.