The latest update supports customized themes. This means you can select how the site looks for you. The way you change your theme is go into your preferences (click on your avatar in the upper right and then on the gear wheel), then click on “Interface” in the list on the left side. Pick your Theme in the Theme drop-down menu!
- Classic GF is the one we have now.
- (new addition after original post:) Dark GF has a black background and white fonts
- Material design is a new one.
- Default Discourse is the default one Discourse comes with.
At some point we could in theory change what the default theme is for everyone, but there is no plan to do that now - the default is Classic GF.