Wow, that's a good point you are bringing up here. We designed the color schemes to be easily editable and expendible. But there is not yet any documentation on that. So let me try to clarify it here and if more questions are coming up, we can add to our main documentation.

Each color scheme is defined by an .xml file located in the Sourcetrail/data/color_schemes folder. To add a new color scheme to Sourcetrail, just copy one of the existing color scheme files, rename it to your likings and start editing.
To see updates within Sourcetrail for the changes that you did, just open the Sourcetrail preferences and switch to a different color scheme and then back to the one you are editing.

Usually the entries in the .xml look like this:

<snippet>
	<background>white</background>
	<title>
		<text>#909090</text>
		<background>transparent</background>
	</title>
	...
</snippet>

But you may also encounter something like this:

<struct>
	<like>class</like>
</struct>

This like element is just a shortcut that denotes that in the graph the nodes for structs should be displayed in the same style as nodes for classes.

If you want to add the solarized color schemes, we would be happy to accept your pull request :)

Added information on how to create new color schemes with merge of pull request #991.

Thanks for so quickly response, I really appreciate your work.