The latest Themler versions for Joomla 4.x/5.x (567 and up) can/will display a 'Class "URI" not found' error when editing a theme. Do the following to workaround the problem until themler is fixed:

 

Insert the following into the themes "Addtional Head HTML" field under the themes Settings page (include the php beginning and ending tags but without the quotes around each):

'<?php' use joomla\cms\factory; use joomla\cms\html\htmlhelper; use joomla\cms\language\text; use joomla\cms\uri\uri; '?>'