Line 117 of style.css defines:

.sticky-header th, .sticky-table th {  border-bottom: 3px solid #ccc;  padding-right: 1em;  text-align: left; }

The problem with this definition is that it also applies to {th} elements in a table nested in a sticky table.

Example:
I have a field (availability_calendar field) that allows multiple values. Fields with multiple values are rendered as a table in the edit form (to allow repositioning using tabledrag). But now the style also applies to my calendar widget.

Furthermore I guess that you only want to apply this to {th} cells in the {thead} row. If I am using {th} cells in rows in the body (e.g. the first column, perfectly valid use of {th}), it will also be applied to these {th} cells.

So better would be something like:

.sticky-table > thead > tr > th, .sticky-header > thead > tr > th {  border-bottom: 3px solid #ccc;  padding-right: 1em;  text-align: left; }