We can 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>