A plain, more modern HTML style for Doxygen
Tell Doxygen about the files for that style as shown in doxyfile.conf. You might need to adjust the paths depending on where you installed that style. When you run Doxygen, all files are copied into to generated HTML folder. So you don't need to keep the originals around unless you want to re-generate the documentation.
that style uses a custom javascript to hack some nice stripes into some tables. It has to be loaded from HTML. Hence you need to use the provided custom header. Since its default content may change when Doxygen is updated, there might be syntax error in the generated HTML. If this is the case, you can remove the custom header (adjust your doxyfile.conf). This has no disadvantages other than removing the stripes.
that_style.css was generated from the scss files in the folder sass. If you want to change the style, use those files in order to have better control. For instance, you can easily change most colors by modifying the variables in the beginning of that_style.scss.