Inspect the element that you want to modify to find the class or id name by right-clicking on the element and clicking inspect. As you find the class/id name of the element(s), you can add some CSS and upload the CSS through Custom CSS. Do not forget to add !important in each directive so that it can override the default CSS.
The solution is not easy to mantain (you need to patch your OJS after each new upgrade).
Using !important is not a good practice because “when everything becomes important, nothing is” (is better call css by order o extend the specificity of your styles).
Backend is not designed to be themed and I think this is a good idea, because keeping backend common for every OJS will let users can feel comfortable when they jump from your OJS to others…
I have to create a stylesheet in the route /style, then I modified the file PKPTemplateManager.inc.php in the route /lib/pkp/classes/template/ adding a call to the stylesheet and finally I have to change the backend.tpl in the route /lib/pkp/templates/layouts/ for the requeriements of responsives pages.