<?php
add_hook('ClientAreaPage', 1, function($vars) {

    if($vars['filename'] === 'clientarea'){ // If you want it on the register page, use 'register' instead
        $currentTheme = $_SESSION['Template']; // Store the current theme
        $_SESSION['Template'] = 'twenty-one'; // Update theme to twenty-one for the specific page
        if($currentTheme !== 'twenty-one'){
            header('Location: '.$_SERVER['REQUEST_URI']); // Reload the page
        }
    }else{
        $currentTheme = $_SESSION['Template']; // Do the same stuff as above, just for every other page and set the theme back to the normal one
        $_SESSION['Template'] = 'six';
        if($currentTheme !== 'six'){
            header('Location: '.$_SERVER['REQUEST_URI']);
        }
    }
});

[Click and drag to move]


​