CSS underlying mashup messes up main mashup

CSS in my underlying mashup:

.edit-sim .BMCollectionViewSupplementaryView, .BMCollectionViewCellWrapper {
position: relative !important;
left: auto !important;
top: auto !important;
transform: none !important;
width: 100% !important;
}

I added a custom class to the collection in my underlying widget (edit-sim).

However, the width 100% messes up my main mashup that contains a collection widget.

All cells in this collection widget (that I gave the custom class 'collection') line out and fill the whole width.

How to add an extra CSS part in this main mashup that puts away the width 100%?

Thanks in advance