Change log:
[ WHMCS theme change log ] : clientareadetails.tpl, css/invoice.css, css/invoice.min.css, css/oauth.css, css/theme.css, css/theme.min.css, header.tpl, includes/sidebar.tpl, js/scripts.js, js/scripts.min.js, js/whmcs.js, oauth/error.tpl, quotepdf.tpl, store/ox/manage.tpl
[ WHMCS orderform template change log ] : js/scripts.min.js, js/scripts.js, js/base.js
You will find all files to update your theme from whmcs 8.5 to whmcs 8.6 in the downloaded package, WHMCS/update from WHMCS 8.5 to 8,6.x folder
