How to uses themes in hass.io

Hello, I found info on the fora to change the themes but they all are based on home assistant and not on hass.io.
Can someone help me to change the themes on hass.io ?

Hass.io is just one way of installing Home Assistant, the only real difference is that it has Add-Ons. Just follow the information you’ve found, or the documentation.

hello,

I strougled to get the selection of themes possible via drop down selection.
Finally found that I had to change the settings in : settings > set a theme
Select “Backend-selected”
As soon this is selected it started working. didn’t found this info and took me a while before I figured this out.