Unless you feel tackling three very, very different problems at once I recommend you treat them separately.
* Functional Design: Think of the right things to do at the right point in your app in a way that engages and works for the user
* Aesthetic Design: Pick the right colors, sizes, margins, fonts, icons to support the functional design aspect (keep it simple, skip illustrations)
* Web Design: Get it to CSS/HTML/JS
It is very simple to get a random aesthetically pleasing thing going (there's a ton of theme resources out there) but for obvious reasons those do not include any consideration to your projects functional design needs, which then trickles down to the lack of strong aesthetic elements and an app identity.
My recommendation depends on the type of person you are and what you are looking for. Do you wanna do self-reliant and do everything by yourself? Think of it as another never ending journey (just like the coding one) and enjoy the ride.
I think tailwindcss is one of the best tools to guide you towards making a lot of right decisions quickly when it comes to aesthetic and web design. Starting keeping bookmarks of whenever you stumble across a page and think "this feels/looks good". Review whenever you wanna build something. Make it easier on you: Skip graphic heavy stuff, pay extra attention that work through great typography, borders, use of colors.
Functional design, well, is super fucking hard and I don't know of any great online resources that teach it on a high level (anyone?). It depends so heavily on the individual problem you are trying to solve and has so much to do with critical thinking, understanding the entire problem space (including user motivation, where they might struggle) and intuition.
To make this part not completely overwhelming definitely work on a problem that you personally have (or at least can heavily relate to), think about that problem as much as you can and figure the simplest possible steps to get you there.
The alternative route would be to work on finding a co-creator, someone who fills in the gaps. Since we are talking about side projects you can absolutely live a little dangerous here and start connecting with people over indie hackers, hacker news or whatever but if you know someone you have worked with before, and feel like you might enjoy working with on a side project go to them first.
good luck :)