Remove reference to the `config` variable in the template files for custom themes

It looks like https://github.com/tomchristie/mkdocs/commit/7167406bc9410410ea7617cd3ed30848636c45d8 broke the various custom themes because `config` is no longer a variable exposed to the templates. This removes the reference to that variable, making the custom themes more in-line with the base theme.

https://github.com/tomchristie/mkdocs/pull/90 will bring things further in line, allowing people using these themes to add this CSS to their extra_css if they want.