Light and shadow, colour theory, and shapes are all things you're going to have to study up on, as well as practice. Now of course going for a whole study of Art can't be done, but game dev/game design/art in general all require a decent amount of studies which can be done if you put your mind to it.
Follow the following playlist for Pixel art basics, animation basics and basic use in Unity.
Drawabox | A free, exercise based approach to learning the fundamentals of drawing
Follow this website for more in-depth knowledge in topics. Covers traditional art studies.
Try and stick to a schedule, or else stuff can get hard to stick to. Assign time and tasks for sessions.
Since you have a decent knowledge of programming, you can start by learning Unity basics to understand the tool and get to know basic stuff.
Unity Essentials - Unity Learn
After the basics, you can try and work on small stuff like making a sprite walk in a room - the sprite being your own and so on.