[WEBUI] Go back to default Grey Theme / Increase font size

[duloo0]

Is your feature request related to a problem? Please describe.
The previous version of Glances had a grey appearance, which has now been changed to black. Please allow us to have a grey based color scheme again.

Describe the solution you'd like
Bring back the grey color scheme from the previous version.

Describe alternatives you've considered
None, just like the grey.

Additional context

[nicolargo]

The current version of the Glances WebUI theme reproduce the style of the TUI (console user interface).

A feature request is open to make the theme configurable: #2048

Any contributor are welcome.

PS: @duloo0 thanks for the issue report, i close it, you can follow the feature request in #2048