The difference between the two variables is that $User.UITheme returns the look and feel the user is supposed to see, while $User.UIThemeDisplayed returns the look and feel the user actually sees. For example, a user may have the preference and permissions to see the Lightning Experience look and feel, but if they are using a browser that doesn’t support that look and feel, for example, older versions of Internet Explorer, $User.UIThemeDisplayed returns a different value.
<apex:page> <apex:pageBlock title="My Content" rendered="{!$User.UITheme == 'Theme2'}"> // this is the old theme... </apex:pageBlock> <apex:pageBlock title="My Content" rendered="{!$User.UITheme == 'Theme3'}"> // this is the classic theme ... </apex:pageBlock> </apex:page>