A custom theme can not only change the appearance of your Totara Learn significantly, it can be branded and styled to look similar to your organisation's website or other systems you use internally. You can read more guidance on developing themes for Totara Learn in the developer documentation.
Totara Learn also has a number of in-built themes which can be enabled and (depending on your selected theme) customised by a Site administrator.
(This video is taken from the Dashboards and basic theming course on the Totara Academy, where you can access more resources and learning materials - including other videos).
The Element library contains a set of sample elements that are used on your Totara site, this is useful to ensure that everything has been correctly themed. You can reach the element library from the Administration block by going to Site Administration > Appearance > Themes > Element Library.
There are options for a number of theme areas, including toolbars, flexible icons, and a pattern library. There is more information for flexible icons in the developer documentation.
Customising HTML email
In Totara 10 a default look was applied to HTML emails, in the form of a surrounding design which includes background colours. If you've got a custom theme then changing those colours, or the whole surrounding design is something you may want to look at.
You can read more about how to do this in the public developer documentation.