I could write a short tutorial here how to do basic styling but I don't know will you gain anything from it. Having done HTML/CSS/SCSS in between my full-stack JS/TS work, instead I'll just list some of the things that I've learned from doing web UI/UX. Maybe you'll get some insight from it, people have mentioned other great tools like Sketch which are great but these are some practical tips.
* Space elements using flex-box. Use position attribute only when must.
* I've mostly ditched classes for Styled Components, but if you do use classes try to get most of the BEM-syntax and SCSS. Try to avoid id and element-wise selectors for styling.
* Use CSS preprocessor like SCSS. Makes your life easier. `&` especially.
* Use margins to position elements in relation to each other. Padding to maintain or inflate element's size. The difference between margin and padding isn't massive, but try to stay consistent in your styling.
* Use (as general advice) light background color with more distinctive primary colors, emulate your favourite sites' style to hone your visual skills.
* Don't overdo it even if the UI widget you found looks cool. For users the novelty will soon wear off and you'll be better off with something simpler and more functional.
* Also don't stress about styling, just maintain a distinct style that is simple and working and you'll be good until you can hire your own designer. Vast majority of start-ups with their fancy UIs are mostly just pretty balloons filled with air. Maybe it attracts some people, but most people (myself included) are happy with simpler layouts as long as they are easy to use. Sure if the site is indisputably ugly it can be jarring but you get the point :). Note that pretty visuals != good user experience.