With the way the Modern Designer is set-up to handle the width of the fields by sliders on the Layout page, it's going to be really complicated to get the table structure resized the way you want via CSS and not have the system undo it when you do things like add/delete rows on the table. It would be much easier to size the columns in the table using the built-in functionality to drag the sliders to resize the columns.
If the issue is that you just want want those two columns not to be so wide and fill the full width of the form, then I would just put an empty Custom HTML element either to the right of those fields within the table, or to the right of the table itself to force them to a smaller width.
Additional side-note: With your CSS that is formatting the header of the table - I would not recommend using those "pdc" class names to target elements. I would recommend adding a CSS Class name to the table and using that to target your CSS, like this (which has myTable as the class on the table):
.myTable table th {
background: #337ab7;
color: white;
text-align: left;
}