label has the highest CSS specificity right now in your CSS files.
You can overwrite this for different areas in your application by for example adding a class to the snippet to create a new definition for the label with higher specificity.
.larger-labels
and then
creating this in your css
.larger-labels label{
font-size: 1.4em; //whichever size you wish.
}
A good way to check which element you need to style is to check in the Chrome Element Inspector (F12 while in your browser). Using this you can look at the HTML layout, and format / style your application accordingly. After you know which object to style, put in a class in your css and add that to the required element.
If, for some reason your element gets overruled, you might want to add !important to the css.
example:
.boldclass {
font-weight:bold !important;
}
Add a class name to the description field in the snippet and reference that in your CSS.
Erik Heddema wrote a nice how-to on how to update your styling: https://world.mendix.com/display/howto50/Setup+Mendix+UI+Framework+with+just+CSS