Changing search fields widths

This is an old question:

https://mxforum.mendix.com/questions/1923/Resize-or-rearrange-datagrid-search-fields

About changing the widths of search fields. It gives the answer I think I'm looking for:

.dijitInline mendixDataGrid_searchItem { width:150px;  }

However, I don't know where to put this (what I am assuming) is CSS. Any pointers?

Cheers.