Remove highlight background color from table cells

[panr]

Provide a description of the task

This a follow-up of #6229

Right now when a table cell is focused, it has a delicate blueish highlight background color and I'm not sure about that...

Actual

This behaviour is mainly inherited from the widget focused element styling (about which I'm not sure either 😛).

Another (small) issue with this is that ATM when the user sets the background color of a cell, the highlight set via CSS will not work... And we have something like this:

Expected

IMHO there's no need to highlight focused elements inside the table.

We implemented the highlight background color workaround in issue #6229, but I think the best way will be to remove background changing. Our focused elements have outlines and it should be enough. But if in some scenarios we decide that outline is barely visible, we can change its width to 2px (Excel and Google Sheets have set thick outlines).

Below is example with thicker outline (where cell has set 2px black border):

Other examples

Here are some examples from Excel and Google Sheets (I chose spreadsheet examples because they are more complex than text editors and for example in Google Docs there is no outline inside tables at all ¯_(ツ)_/¯ ):

Excel (just solid green outline and no highlighted background for selected cell):

Google Sheets (thick solid blue outline and no highlighted background for selected cell as well) :

[panr]

#6446 (comment)

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