How to Identify Salesforce User Experience Theme in Visualforce Page?

Use User.UITheme and User.UIThemeDisplayed in Visualforce Page to determine User Experience Theme.

User.UITheme : Returns the theme that is supposed to be used.

User.UIThemeDisplayed : Returns the theme that is actually being used.

User.UITheme and User.UIThemeDisplayed will return following values.

Theme1—Obsolete Salesforce theme

Theme2—Salesforce Classic 2005 user interface theme

Theme3—Salesforce Classic 2010 user interface theme

Theme4d—Modern “Lightning Experience” Salesforce theme

Theme4t—Salesforce mobile app theme

Theme4u—Lightning Console theme

PortalDefault—Salesforce Customer Portal theme

Webstore—Salesforce AppExchange theme

Sample Code:

<apex:page>

    <apex:pageBlock title="Theme">

        {!$User.UITheme}

        {!$User.UIThemeDisplayed}

    </apex:pageBlock>

</apex:page>