Is there an easy way to customize the existing sphinxdoc theme? For the default theme, there are many theme-attributes, but in sphinxdoc I can\'t even set a log
Unless I misunderstand you, the standard Sphinx documentation tells you how to modify existing and create new themes.
I actually installed the Sphinx cloud theme, and then started editing its templates; so I had a new theme where I could see exactly what is required, but that I didn't need to create from scratch.
If you want to alter the CSS layout, you can add CSS files (or images) into the _static subdirectory of your source, and edit your conf.py as necessary. Again, the cloud theme was my best example for that.