GHA: Navbar UI/Styling issues with bootswatch theme (bootstrap4)

[MadhulikaTanuboddi]

301-bs-themes and 302-bootswatch-themes app's shinytest tests are failing.

This is happening with almost all the themes using bootstrap4.

The navbar background color being the primary change or difference, there are also font styles and text color differences with the nav items.

Check the following few examples from the screenshare while running the shinycoreci::fix_all_gha_branches() (ran on 10/27)

https://user-images.githubusercontent.com/5369655/139154734-9d643d81-a7d9-4c3a-9702-dc8a3e98003f.mp4

https://user-images.githubusercontent.com/5369655/139154805-3c55de0f-e625-4e34-87c1-16e0f10c42de.mp4

https://user-images.githubusercontent.com/5369655/139154824-c78ff202-fa1e-4315-899d-207d447f6dc3.mp4

https://user-images.githubusercontent.com/5369655/139154879-55fa5702-4194-4fc7-b584-c735ae4fdc36.mp4

[MadhulikaTanuboddi]

These issues are fixed.