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)
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
- 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-qamanual pipeline job)