Fix UI problems on Lepton Theme Demo #13942
Description
Resolves #13941
Fixed some UI problems. Also, I have created a new issue (#1433) to generally revise the lepton-theme demo.
Checklist
- I fully tested it as developer / designer and created unit / integration tests
- I documented it (or no need to document or I will create a separate documentation issue)
- I worked with the design team to get their idea for UI/UX design (or no need to design for this PR)
- I've assigned reviewer, related labels and set a milestone for this PR