Use new Qt brand colors at doc.qt.io

The old brand colors used in the light mode clash with the new graphical elements, so use the new brand colors in documentation (at least in the light mode).

https://www.qt.io/brand/development/brand-colors

You can see an example of how it currently looks here:

https://doc-snapshots.qt.io/vstools-dev/index.html 

Color suggestions from Negin for the dark mode:

• Background: 2E2E2E
• Table border: 3A3A3A
• Link: 23B26A

Light mode:

• Link: 1F9B5D