💻 Slides restyled

[Ano-M]

Fixes #6437

• Slide headings fonts changed to balsamiq sans (bold)
• Gradient background changed

How to test

• Go to any teacher slides page (e.g., http://localhost:8080/slides/2#/2)
• Verify that the headings display in blue with the Balsamiq Sans font
• Verify that the body text is readable in black
• Verify that the background displays a subtle gradient
• Check that slides render correctly on different screen sizes

New:

Old: