You can edit your email templates under Settings > Emails > Workflow. To customize the HTML and CSS of your emails, click on the "Code View" icon (it looks like this: </>). Note that not all email clients will support different styles, so make sure to check the compatibility of what you would like to edit. Here is a link to a useful tool to check compatibilities.
Note that we only support inline CSS, so, for example, we are unable to upload new fonts.