@Paul_Bovbel I found a way. 
It’s a bit of a hassle, but if you clone the rclcpp repository and manually run doxygen, then you get the standard Doxygen theme.
It even has the inheritance diagrams:
For packages without a doxyfile, I did a quick test by copying (and slightly adapting) the rclcpp doxyfile and it also generated those API docs just fine.
Regards,
Johan
EDIT: i.e. manually running doxygen, as opposed to the standard way of building package documentation (rosdoc2 build) which also runs doxygen but applies the other theme to its output.