Most people often attribute design to making something pretty, or using photoshop, or tweaking a color, true design is about functionality not aesthetics.
At the core of it design is functionality done right. Most of my engineering/programming/logical friends look at me strangely when I try to explain this but it is true.
It's about human interaction, what is your desired response, how will people react to this, how will it be used, etc.
Color theory is only important because it invokes certain emotions, font's because it conveys they tone you are speaking in, using lines and the other elements of design all work together to lead they eye and evoke a desired response.
There are a lot of things out there that are considered good design but are ultimately useless in achieving their goals, these are simply works of art done in a designed style and not true design.
It's about purpose and intention.
I'm willing to bet you know more than you think you do, that you've been exposed to so much design your whole life that your guesses would be more educated than you think, and that coming from a background in software your ability to run tests (like A/B tests) would be invaluable.
To conclude, everyone thinks that design is art but really it's psychology combined with rapid iteration(thumbnail sketches) and lots of tests (color swatch tests etc) based on best guesses. That and most great design just rips off something else anyways.