As the title says:
Overridable templates to incorporate into themes, so the look and feel of your home assistant could be styled exactly as you would like to by choosing your own HTML/CSS and Javascript.
Currently it’s not possible to fully 100% theme your home assistant, but only override a bit of CSS using card_mod, which feels a bit like a hack.
This is actually possible using card-mod themes.
See https://github.com/thomasloven/lovelace-card-mod/wiki/Card-mod-Themes
See the examples here for how customisable it really is: https://github.com/thomasloven/lovelace-card-mod/releases/tag/2.0.0