All of the follwing is done inside of a .css file.
Here's an example for the CSS used to style the text seen inside each button.
The class is called .topic and is used inside of class="" attributes to affects tags in .html files. Putting a period before a class you are naming is essential.
The same is used for my .bocks class used to make all the Blue Bordered Boxes this text is placed in:
A period is not needed if you are affecting all of a certain tag, for example this CSS that affects the <body> of all my webpages:
The same is applicable for affecting all <p> or <footer> tags placed in your .html file.
To utilize the CSS you've made, link it to the desired .html file(s) and attach Classes to tags you want affected.
Writing <div class="x"> Affects all content inside, unless more Classes are used on specific elements inside (either more Classes on further <div> tags inside, or Classes inside <span> tags). </div>