• Doing a black line stroke inside of text is not a good thing to do, don't do it!

What's your reasoning? Subtitles on video have a black stroke, and that's text.

I didn't emphasize enough that the key word there is "inside".

A black stroke on the outside of text is often a simple and good way to increase contrast, yes, but when its an inside stroke at a smaller font size, or a font with weird serifs, it will interfere with the width of the colour fill on the characters and make the font look really different from what is intended:

[attachment=34757:text_example.jpg]

The outside stroke example isn't perfect for various reasons, but it's much more readable.

I started experimenting with changing the graphics. I mainly took the advice to try and find a overal style. Luckely for, me the number of assets is not that big and I can use opengl shaders for quick experimentation. Thanks for giving me pointers and feedback, this only encourage me more to 'find a style'. It will take me some time, but I will definitely post progress every now and then. Probably also some failed attempts, just for fun.

Not sure about goiig 3d though, I will probably stick with 2d. Smooth animations and lighting are not that hard in 2d, I am not using sprites for animation and movement. And I need the performance to render huge numbers of units on low end hardware.

There're a common pallette of colors that you can see everywhere and some of these are awfully abused by movies: blue and orange. You can also see red and yellow, primary collors and white, whites blacks and maybe a primary color... it all depends on the context too... red and green if it's awful on purpose. Shadows are bluish and lights yellowish/reddish. Hue, saturation, and luminosity are your friends.

Similarly make the shadows of helicopters and other aircraft further from their source object the higher they are, so there's a perception of "descent" and "ascent" as they land or pull away.

This is already something I tried to implement. You see this in action in this video, is this what you mean? At the start of a level, the helicopters fly in from random directions and descent.

There's lots of good advice here, but I would also ask, at what point did you make this decision?

I learned that the graphics are not good enough for a release.

If you came to this conclusion as a sort of justification for not making it through greenlight, but previously thought the art was acceptable, then I think it's worthwhile reconsidering what you would define as "good enough", relative to what your goals are. Consider that lots of games released on mobile look much worse than your game does. All of my "programmer art" is much worse than what you've got. If your goal is to make a "success" that appeals to as many people as possible and serves as a source of income, then yeah, you'll want to make improvements everywhere you can- and hiring an artist is the ideal option, as was mentioned. But if your goal was to make a portfolio piece - you've accomplished that. If you goal was to make something that's fun, and the focus was never on the art - then (I haven't played your game, so I'm assuming) this is also a success. If your goal is to get the experience of going through the full process of making a game, from writing the code to selling it on a store, then maybe you just need to switch to a more open platform (mobile?), but you can still do that with this level of art.

It's important to keep in mind that Steam (greenlight included) is flooded with content of wildly varying quality. It's a platform where marketing values and visuals win most of the time, regardless of the quality of the game.

That's not an excuse to drop the art issue though, if you want to improve it. Just make sure you're doing it because it fits with your goals, instead of doing it because people on greenlight said you should.