bat --version on my old mac (OSX10.11) gives 0.6.1, so no wonder why it does not work for f90… Now I tried brew upgrade bat, but the compilation of dependent libraries (here rust) failed. So, I switched to the latest macOS (Monterey), then everything worked fine out of the box (like “less with color”) 
(Indeed, I would like to migrate to Monterey asap, but not finished yet…)
Because bat has a lot of options (like different themes), I will check them for more customization. Thanks!