What is the best Approach for CSS framework of an Enterprise Cloud application?

There are several ways to style the elements in each page, in Enterprise applications usually the CSS Framework size increased about 1 MB, and when your users are using slow internet connection, you should decrease css framework size.

we can create new CSS for our element like .Blah and value it in css framework and do this for each element which cause increase size of css framework, but a cleaner page

<div id="blah" class="blah"></div>

we can also use our css framework utilities in each view to format each element to keep size of css framework, but a non-clean page

<div id="blah" class="margin10 padding20 bg-color-red fg-color-white text-bold else"></div>

which of above approach is the best approach for an Enterprise application, while you ensure that a majority of your users are using slow internet connection