Feature request: custom theming

[yinchi]

It would be nice to be able to set the edge and text color. Currently, trying to embed a DBML diagram on any dark-mode webpage makes the blue relationship information text almost disappear.

Perhaps the syntax can be like:

Theme {
    background transparent
    table_title 000066
    table_title_text white
    edge orange
    edge_text red
}

plus any other parameters that are used to set up color in the intermediate .dot files.

Alternatively, a set of built-in themes such as "light" and "dark" would also work.