@MJ CSS? Nah. Just execute some JS
For example:
$(window).on('action:widgets.loaded', function () {
$('li[component="categories/category"]')
.add('li[component="category/topic"]')
.find('.stats > small')
.each(function (i, el) {
var
$small = $(el),
text = $small.text(),
$icon,
iconClass
if (text === 'Topics')
iconClass = 'fa-group'
else if (text === 'Posts')
iconClass = 'fa-group'
else if (text === 'Views')
iconClass = 'fa-group'
else
return
$icon = $('<i class="fa fa-fw ' + iconClass + '"></i>')
$small.replaceWith($icon)
})
});
P.S. Actually, the best way to change something in a theme - is to fork the theme and make whatever changes you want in your own repository. But you'll have to keep track of any new changes from the original theme repository, resolve merge conflicts, etc. This way is a hard way. The example I've provided above is the easy one.