Per the latest release notes, I should be able to apply conditional formatting rules to string dimensions, but I’m still only able to do so for numerical dimensions. I would expect that leaving “apply to” as “select fields” would allow me to type to select a string dimension from the table, but it does not appear to be possible on Looker 26.0.28.
Looker now supports conditional formatting rules for string types in single value and table visualizations.