Insert button not visible

Hello,

Using Tabs & Accordions on Joomla 4.4.1 and Virtuemart 4.20.x

When adding a Tabs & Accordions in my Virtuemart Article, the Insert (or Close/cancel) in the popup are not visible. I see in the debug that adding this makes the button visible again:

.modal-footer {
z-index: 999999!important;
}

I only don't know where to place it as a custom.css part, so a next update does not overwrite it.

Regars Michel