Ok, so I do understand that this might not be super-obvious for some people so I’ll give a little explanation. When it comes to styling every browser has built-in styling for focus called the outline. It looks like a dotted line in some browsers and like a blue or yellow border in others. It is possible to remove this outline with CSS. Do not do that! You might argue that this is enough styling for focus but sometimes the outline is really hard to see (I’m looking at you Firefox!) so a good rule of thumb is to apply that styling you put on hover on focus as well.
People often argue that they do not have the time or budget to care about things like accessibility. When it comes to thinking about focus those arguments are simply not valid. It takes you a couple of seconds to fix this. Just do it!