Missing css rule for the attribute/value scope='colgroup'

[lcarcone]

The file https://www.w3.org/assets/website-2021/styles/core.css contains the following rule:
th[scope=col] {
background-color: #024488;
color: #fff;
}

We have some pages that also use the value 'colgroup' to specify that a group of columns is a header and they are rendered differently.

Could we add the value 'colgroup' to the rule above ?

[vivienlacourba]

Relates to #648

[vivienlacourba]

This should not be necessary if the change suggested in #648 is implemented.