The line width of the element's frame can be changed by using the width buttons on the "Style" panel after having selected the layers to apply the change on. "0px" removes the line, "1px" draws a single-pixel wide line (the default), "2px" a somewhat thicker line and so on.
"Simple" is the normal draw mode while "Marked" draws a cross on each vertex of the element. The cross size is constant so the shapes stay visible even on large scale where the elements would otherwise become single pixels.
A layer can be configured to draw a diagonal cross on rectangles. To enable this style, select "Cross" from the "Styles" toolbox. To disable it, choose "No Cross". The cross drawing will add to the fill pattern and applies to rectangles only.