Hi Duncan,
sorry for having done a little mess. Yesterday I was a little tired and I copied the code in a wrong way. I noticed it only now by reviewing your post.
In line 539 (more or less) change this
$category = get_the_category();
$category_name = $category[0]->cat_name;
$category_slug = $category[0]->slug;
// Recovering Saved Color Values
// Define the Settings for recording
$cat_var = "cat_".$category_slug;
to this
$category = get_the_category();
$category_name = $category[0]->name;
$category_slug = $category[0]->slug;
$category_name1 = $category[1]->name;
$category_slug1 = $category[1]->slug;
$category_name2 = $category[2]->name;
$category_slug2 = $category[2]->slug;
// Recovering Saved Color Values
// Define the Settings for recording
$cat_var = "cat_".$category_slug;
$cat_var1 = "cat_".$category_slug1;
$cat_var2 = "cat_".$category_slug2;
In my previous post the first line was missing.
Then, on line 575
$featured_style = "style='background-image: url(".$url."); background-size: contain;' class='featured-image ".$animation_style." ".$cat_var."'";
into this
$featured_style = "style='background-image: url(".$url."); background-size: contain;' class='featured-image ".$animation_style." ".$cat_var." ".$cat_var1." ".$cat_var2." '";
Sorry for the mistakes. This should work.