Depending on exactly what changes you want to make, you can also target categories through the class added to body on each page. eg.
<body id="page-course-view-topcoll" class="format-topcoll path-course path-course-view gecko dir-ltr
lang-en yui-skin-sam yui3-skin-sam localhost--moodledev pagelayout-course course-2 context-25 category-1
Aug06 showforsemester drawer-open-left ">
Then in your css (Custom css setting or Raw SCSS depending on the theme used) - obviously the css here is just for illustration.
.category1 #region-main {background-color:#033;color:#fff}
So, if you are looking to make a few css style changes (and no changes to renderers/layout files/templates/etc), then you may not need to clone/build a child theme at all, simply target the appropriate css as required.