This really depends of what type of category page you chose, for example for categories with topics the code will be something like this:
.category-list.with-topics .category-name {
font-size: 25px;
font-weight: normal;
color: gray;
}
You need to use the browser’s inspector to find to CSS class (and parent) that you want to use, see
The parent serves to indicate that you want to change the appearance of the category name only on this page and not globally.
To change the font type see How to change default font of your site
The next time before posting use the search function on the top right and you will surely find the answers you are looking for.
This one instead, CSS Tutorial, is a good place to start learning a bit of CSS.
