Table Header text color isn't set correctly when a thead is present

[b-nik]

Normally, when a tr has a color class let's say "is-link", the text will get the invert color so it's readable.

However, when we're inside a thead, this doesn't happen because this line https://github.com/jgthms/bulma/blob/main/sass/elements/table.scss#L161 seems to be setting a single color for all cases.

I can set the $table-head-background-color and $table-head-cell-color variables and that's fine - but I feel like it should also work with specifically setting a class on tr regardless of thead existing or not.