Hover tooltips in the UI are hard to see due to transparency on the grey background

[whisperity]

Describe the bug
The hover-on tooltips in the web UI are hard to see, as there is too much transparency in the grey background with lots of whites in the background.

CodeChecker version
131861b

whisperity@476dc9a425ff:~$ CodeChecker-Release/bin/CodeChecker version
CodeChecker web version:
------------------------------------------------------------------------
Kind                          | Version
------------------------------------------------------------------------
Base package version          | 6.15.0
Package build date            | 2020-11-13T00:49
Git commit ID (hash)          | 131861b7e94298f58cad883a533814573b89a0f3
Git tag information           | 6.15
Server supported API (Thrift) | 6.35
Client API (Thrift)           | 6.35
------------------------------------------------------------------------

To Reproduce
Steps to reproduce the behaviour:

1. Hover on any ❔ on the UI.

Expected behaviour
Better readability. I have changed the background-color: attribute to rgba(0,0,0,.9) from rgba(92,92,92,.9) I think, in browser webdev tools and achieved the result below. It's not perfect still, but certainly more readable.

Desktop (please complete the following information)

• OS: Windows 10 19.09
• Browser: Firefox
• Version: 83.0
• Screen panel type: I tried it both with TN and IPS panel LCD screens. Connections are via HDMI. The greyness is actually worse (i.e. the tooltip is less readable) on the IPS panels (which are usually regarded as better in terms of colour by both gamers and art/animation/visual workers)