you have some lines of repeating code (the properties that are the same for both pills), so I’d give them a class="pill pill-1", and put all the repeating properties in the .pill class
instead of border-radius:50%, I’d give that a really high px value (like 1000px), that way they look more like actual pills
if you put the transition:0.5s into the .pill selector instead of .pill:hover, they’ll also smoothly move back when you stop hovering
I wouldn’t use float, that’s kind of outdated CSS. Instead, set the .pills class to display:flex with justify-content:space-between or space-around
to make them look even more realistic, you could apply a linear or radial gradient instead of a uniform background-color
That’s pretty much it. As for articles to read, I’d recommend to just keep building little projects and look things up as they occur. Generally, a good resource for CSS: https://css-tricks.com/