The 'legacy' theme is in an early stage of development, so it is not as developed as the 'default' theme. The experience with the 'legacy' theme may not be as consistent as using the 'default' theme.
This option is only currently available to users signed up to beta updates. To become a beta tester, see the article: How do I become a beta tester?
- Go to the Settings menu and select User preferences
- Choose the theme from the drop-down menu
- Select Save and reload