osx mojave beta is available to the public. To get it you have to sign up at apples btea program website.
Once signed up, you can click the link for the osx mojave beta, sign in and then can download the beta from the app store.
It takes a while to download and install, and its not really rteady for production use, but if you are ok with a few bugs / issues, like me, then its very nice to get a change of operating system
The major chagne with this version of osx is that we have a full night mode! (and a ton of other features that I’ll be making posts about, including iphone and ipad apps on the desktop!)