You can use the Theme Editor to edit the individual CSS and PHP files which make up your theme.
Begin by choosing a theme to edit from the dropdown menu and clicking Select. A list then appears of all the template files. Clicking once on any file name causes the file to appear in the large Editor box.
For PHP files, you can use the Documentation dropdown to select from functions recognized in that file. Lookup takes you to a web page with reference material about that particular function.
After typing in your edits, click Update File.
Advice: think very carefully about your site crashing if you are live-editing the theme currently in use.
Upgrading to a newer version of the same theme will override changes made here. To avoid this, consider creating a child theme instead.