Then on the Report Attributes tab, I edited an ordinary (i.e. non-link) column, and typed magenta150 into its Column Formatting ... CSS Class field. Running the page to see the report, that field is displayed 150 pixels wide with a magenta background. So far, so good.
I then edited my link column, added the reference to the magenta150 class in the same place... and nothing happened. When displayed, the link column is not 150 pixels wide, and has no magenta background.
Solution: When editing the link column, don't put anything into the Column Formatting ... CSS Class field.
Instead, put this into the Column Link ... Link Attributes field: class="link150"