luci-theme-material: update cascade.css to use first-of-type instead of last/first...

luci-theme-material: update cascade.css to use first-of-type instead of last/first-child

first-child:
Selects an element that is the first child of its parent. The element must
be the very first child of its parent, regardless of its type.

first-of-type:
Selects the first element of its type among its siblings. The element must
be the first of its type (e.g., <p>, <div>, etc.) within its parent, but it
does *not* have to be the first child.

Makes the CSS usage more robust if the layout changes and new html elements
are added.

Signed-off-by: Florian Eckert <[email protected]>