1. Become very strong at raw CSS.
2. Learn how to use your terminal.
3. Start making "polish" PRs in whatever site / app you work on.
4. Pick up skills from engineers organically around you as you clean up the front of the frontend.
5. Learn that design is meant to move, and pick up animation!
If you really learn CSS, like becoming the best person at your company in it, you will become immensely valuable to your team. Every great coding designer I've known got there because they were frustrated their engineering team would miss details from their designs. They learned how to clean up behind them, and then next thing they knew they were delivering coded prototypes, and not designs to their feature teams. They knew static designs would fall apart, and had the stamina to work on the three issues that come up most in web design: text truncation, focus issues and media query issues.
Source. Coding designer that's been at it for 25 years. I've worked at every level of design: from grunt, to open source system-designer, to Head of Design at a public company for a 45 person design team. Gave it all up to get back in the code and hack!
Coding designers are the best! Pair them with a strong visual designer and a patient engineer and you will have an awesome product.
Side note. I've worked in dozens of front-ends over a long career. CSS is the only constant.