Custom Menu URL Removes Symbols

I’m taking advantage of Theme Switcher plugin to make this site more versatile. This is done with the url parameter *”site.com/blog/templatepage?wptheme=theme_name”*

Problem is if a visitor visits the site via a link with this parameter then every time they revisit the site that theme will load, not the default.

So I want to create a menu with custom links that use the get_current_theme() function to append the current theme name to the end of all links in the menu.

I have this in each template <?php $theme_name = get_current_theme(); ?>

So I want to do something like this for the custom menu URLs;

https://www.site.com/blog/templatepage?wptheme=<?php echo $theme_name ?>

Problem here is that every time I create/save the custom menu item it removes the brackets <,>
How do I fix this? Maybe somebody has a better idea which will let me handle theme management?