The difference between the $User.UITheme and $User.UIThemeDisplayed is that $User.UITheme variable returns the look and feel the user is supposed to see, while $User.UIThemeDisplayed returns the look and feel the user actually sees.
Sample Visualforce Page Code:
<apex:page standardController="Account">
Theme is {!$User.UITheme}<br/>
Theme Displayed is {!$User.UIThemeDisplayed}
</apex:page>
Output: