Fix ide-tree-header button background

What does this MR do?

This MR fixes a minor styling issue for Web IDE where <button> has a grey background on Firefox

Screenshots (if relevant)

Before	After

Does this MR meet the acceptance criteria?

• [ ] Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] API support added
• [ ] Tests added for this feature/bug
• Conform to the code review guidelines
  • [ ] Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • [ ] Has been reviewed by a Backend maintainer
  • [ ] Has been reviewed by a Database specialist
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conform to the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?