In Reason 9 and later and in Reason Essentials 9 and later you can choose from three different visual Themes: Default, Blue or Dark. The Themes affect the Browser and Sequencer areas, whereas the Rack and Mixer areas are left unaffected.
Here is how you change the Theme:
Open the Preferences dialog (from the Reason menu on Mac, or from the Edit menu on Windows). On the General tab, select the desired Theme from the list:
After this, restart Reason/Reason Essentials.
The new Theme is now active:
The picture above shows the Blue Theme.