How can I style Javascript with CSS?

You can't directly alter the appearance of alerts with vanilla CSS, but you can find way to do this with JQuery or more robust CSS libraries (I think Bootstrap allows for custom alerts). This is probably a little advanced for your current project, my recommendation would be to take the DOM manipulation course as was suggested above, document.write is an easy way to get things on the screen, but it's not how most JS is added to html. After you master the DOM course, JQuery is another great way to add elements to the DOM, it's also widely used to manipulate classes and styles of elements in your html. Appending elements to the DOM will allow you to use selectors much easier, however I believe you can add a class to your document.write statement if you really want to:

.addedStyle{
   color: red;
   padding-top: 5px;
}

document.write("<p class="addedStyle"> Kis cuuki <3</p>");