Styling for Tooltip and side nav on dark mode needs fixing

[huijing]

Describe the bug

Tooltip blends in with the sidebar (both of them have the same background color - maybe the tooltip needs some depth + I’ll add an attachment)

To Reproduce Steps to reproduce the behavior:

1. Go to 'https://webmonetization.org/docs/intro/web-monetization-flow/'
2. Hover over a tooltip
3. See described situation

Expected behavior

There should be more distinction between the border of the tooltip and everything behind it.

Screenshots

Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Additional context Add any other context about the problem here.