chore(demo): add position sticky to category's title

[amtins]

Description

This allows the category title to remain at the top of the page when scrolling down.

Changes made

• adds the same background color as the body
• removes the title margin
• adds padding at top and bottom

[github-actions[bot]]

Coverage report

St.:grey_question:	Category	Percentage	Covered / Total
🟢	Statements	98.36%	481/489
🟢	Branches	93.42%	227/243
🟢	Functions	100%	137/137
🟢	Lines	99.15%	464/468

Test suite run success

142 tests passing in 9 suites.

Report generated by 🧪jest coverage report action from 729e6cb1f1c2c311affdac822988569b0a0c3973