After using this for a few days, one or two additional comments: firstly, this is generally really a good theme: it is clearer, more modern in appearance and easier to use than the default theme. I should rather like to use it as the default theme for Experimental in the future (I presume that this is released under the same Artistic Licence as the rest of the Simutrans project)?
I have already commented that the background colour could do with being a tad darker. One other thing: there is no shading showing which tab is currently selected, which makes using tab interfaces (such as the depot window) unintuitive. Other than that, there is much to be said for this theme, suitably modified, becoming the default theme of Simutrans: I suspect that its UI style is more familiar to most users and therefore likely to make Simutrans appear more user friendly.