Discourse newly introduced theme selection option for individual users. That means that you can choose the theme you like from the list of themes provided by the site.
Got bored of dark theme ?? Try the new Material Design.
Go to Userprofile > Preferences and at the bottom, you will find the option to select the theme which you like!!!
Still did not get what to do ??? Don't Worry we got you covered, check the GIF out to see how to select a new theme.