Get yourselves some artistic sensitivities and whole bunch of cohesive ideas because you'll need both in huge amounts. It's one of those conditional capacity that even the best of modders can't miss even in small quantities.
Your results will be directly caused by how much thoughts you put in. You need a plan. Predictable, designed, composite of decisions, reasoned.

90% of gameplay is context, its appeal is found in soooo many assets that even a splash of colors wrongfully dumped on the UI will turn off interest in as many ways as you can think about; YOR is a magenta Robot while Drengin is bloody red.

There are reasons for this -- symbolic enough to stir an impression (scratch this, EMOTIONAL state of minds), generalistic enough to provide racial identities (i'll insist - DIFFERENCE in definitions), artistic enough to please the Sci-Fi crowds (through deduction - even others).

10% of gameplay is YOU and how you interact and react to whatever happens to be presented.

You wanna know what defines GC2 graphics and its context (partly)?

Look at these...

Colors, Logos, race abilities and what political party they use.

Some Minor adjustments;

Each and all of these elements are moddable.

Without variations you aren't modding.

Choose.

5- Tech trees