I am good with CSS, yet, I feel my design skills are terrible. My web apps look too basic and lack aesthetic quality.
I am skilled in a multitude of technologies, including HTML, CSS (SCSS and SASS), JavaScript, NodeJS, PHP, and SQL. Yet, I feel my poor designs are ruining everything.
inspiration aka borrow/steal ideas
pay close attention to things you like or dont like about sites that you visit and apply those ideas to your own.
try to replicate features individually, as in just build a hamburger menu without anything else.
build lots of small components regularly then put them together in bigger projects instead of trying to make the whole thing on the first go.
I would suggest that you do this, but start with your own project that you slowly build as you draw in inspiration. Do not go directly to component design. Slowly build and improve this thing over time finding out which visual elements are lacking. Look to other web designs as examples of what to add, or what to remove. This process allows you to learn not only what elements go into a website but why certain elements are repeated.
Then, I suggest you slice up this page/project and design those individual components separately making them more robust and well designed as individual components.
On sites I like to visit, I generally inspect their pages and view the source code. In fact, www.alaskaair.com has a fun Easter egg in their source code! 
I like the design of my bank, www.wsecu.org. So yeah. Spend time looking at the actual elements of pages you like or sites you find interesting.
An app I also enjoy is Dollar Shave Club. Their app is highly professional and simply fun to use. Perhaps I’m in the minority here, but I hate Amazon’s website. It’s too cluttered. You simply need to get familiar (as do I!) with more CSS/JS designs and maybe watch some tutorials that help with design. This page might help you: http://www.csszengarden.com/.
It depends on how you actually learn, whether it’s by reading or observing or by doing and getting feedback. You can do both, and it will enhance your desire to move forward and onward as you master skills you have learned. It’s important to remember to never stop learning and growing if you wish to be successful.
You should learn about UX/UI Design. I will give you references about UX/UI :
If you’re looking for inspiration about web design when you don’t have any ideas what makes web design looks good you can get the thoughts that might help you.