Fix layout

[vvtimofeev]

1. On responsive breakpoints, the header menu breaks the layout and the whole page gets a horizontal scrollbar.
2. Native scrollbars look bad on desktop, as if something is broken; we need to choose and integrate a library on the landing that renders a cleaner-looking scrollbar.

Viewport: 1312×848 and slightly below full browser window height.

[SomedayToday]

Hi! I’m new to open source but would love to try fixing this. I have experience with HTML/CSS and can integrate a lightweight scrollbar library. Could you assign this to me or share any additional preferences (which library to use, etc.)?