web design: how to have a better sense of designing

I am into web development. my mini goal right now is to make a blog using flask and mySQL. I learned Java at university and learned python recently.

naturally, I learned HTML, CSS3, SCSS(SASS) and still a beginner at javascript. since the only two things left are a database and a framework to accomplish this mini goal (both take no time to pick up as far as I know especially that I have a programming background), I decided to focus on web designing(my weak-point) a bit more and play around just to grasp the idea

the thing is, after learning the basics of html and css, I decided to start making websites, because I heard the best way to practice was making them. I basically would search for (minimal design) or (website design) and would reverse-engineer the ones I liked. using this method I learned I a lot and found many shortcuts and tricks to do cool stuff (sass, jquery, ajax). .. and here comes the problem .. I HAVE NO SENSE WHATSOEVER IN DESIGNING WITHOUT A PREMADE PICTURE!

even though I can mimic the ones I find, whenever I make a copy from the site I made and try to "play around with designing" I end up making super ugly websites with extremely ugly colors. COLORS are my main problem because I have a big collection of layout in my head thanks to making websites.. but goddammit colors ..

how to have a "better" sense of designing? how do I determine if these two colors are good together or not? I know about color theory but still the shades I end up choosing are too horrible to look at

I started convincing myself to give up and focus on back-end now since it is useless already. but I just wanted to ask as the last resort before giving up completely