CSS theming support

[brodokk]

Hello,

I wanted to have a custom text color support but after some research of how svgbob work I figured out that it's a problem from the rust library and not this project. While I could see for add support over there I notice that, at least for html rendering, the theming of the svg didn't work via an svg in an image but work with an inline svg I decided to try and fork the project.

I read the contribution note and instead of create a pull request directly I'v open this issue. Now I have worked on an implementation on my own (See directly the diff on my fork here: main...brodokk:svgbob:css_theming_support) but I feel I have a problem with it. While the support of custom theming via css work fine I encounter an issue with the support of the align option. Since I'm trying to user a container to change alignment of the svg I'v tried to use the node container of sphinx but it doesn't seems that I can add a custom parameter to the div without creating a new node.

So my solution was to add a custom container node, div, but now the problem is the tests are not working anymore while the command make man work fine. I suppose the test are also not correct, so I've patched the tests for make it work.

Another thing, is this normal that the builder for latex and text are broken without my modification? Are they supported?