They are not errors, they are notices.
Notice: Undefined index: accent_color in ******************/wp-content/plugins/t4p-core/shortcodes/class-button.php on line 328
If there is any PHP error, you will only see a blank page on your website.
and it will begin with PHP fatal error: ……
That notice won’t appear if your shortcode uses that variable. Which means if an accent color is selected.
Those notices are up to the theme author to decide if he wants to define the variables, such as by writing
$accent_color = '';
It doesn’t affect the outlook of the website.
I understand that it looks cleaner if those notices don’t appear.
But, I am just the support guy and not the theme author. It’s up to him to decide if this is one of his priorities.