You should edit your stylesheet.
Within the stylesheet are definitions for how every element of you pages looks.
You will simply change the parameters to your taste..
font-size
font-family
text-decoration
etc..etc..etc..
So, if you right click on your webpage while using Chrome browser, and select "Inspect"..it will show you how it is currently configured, and which line number in the stylesheet where that definition is.
If you are using the DefaultClean theme, it is located here in your website:
/Themes/DefaultClean/content/styles.cssOpen it with notepad and find the element you want to change:
I use "Inspect" and see the "Categories" navigation menu title is this class:
.block .titleSo I find it in the stylesheet:
.block .title {
background: #55372f;
padding: 10px;
font-size: 15px;
color: #fff;
text-transform: uppercase;
border-top-left-radius: 5px;
border-top-right-radius: 5px;
..then change or add or remove definitions...