Below is a sreenshot of a regular old C file. I highlight the function names in bold yellow, the C control flow keywords in bold white and the strings in a sky blue. I find no need to highlight words like 'extern', 'int', or 'typedef'. Highlighting those words wouldn't guide my eyes to what is imporant in the file.
Below is the top part of a C header file.
Finally, I show the bottom part of a C header file. I like the way that the function name in the prototype is highlighted and not the return type.