A very simple option is to use inline html. It allows you to add html elements to your Simple XML dashboards, e.g. for help texts or other custom objects.
Previous Splunk versions used to have HTML dashboards, but they were deprecated. You should check out splunk dashboard studio for the intended follow up solution.
CSS can also be applied inline in HTML style tags. The proper method is to use the stylesheet attribute of the dashboard / form tag and refer to a stylesheet file. Find moredetails here: https://docs.splunk.com/Documentation/Splunk/latest/AdvancedDev/UseCSS
