If you can use a text editor and can copy files from your personal computer to your website then you can use this plugin. Most modern templates for Joomla allow the site owner to upload their own customised CSS file, a file that is automatically detected and used by the site template. The standard Joomla site template (e.g. Protostar in J! 3.x or Cassiopeia in J! 4.x or J! 5.x) allows this. For those templates that do not provide a mechanism for including user-written customised CSS another way to make these kinds of changes is to modify the template. The problem with modifying a template is that these changes may be lost if there is an update to the template or to Joomla. This plugin solves that problem by allowing you to add your own custom CSS file to any Joomla website, regardless of the template that you may be using.
Easily add custom stylesheets to the frontend of a Joomla website with Add Stylesheet plugin for Joomla.
English (en-GB) but it is not difficult to translate the output into any other language