Remove old frame styles

Delete old, obsolete frame-related sass files and styles and replace
them with a new file for menu styles.
Add styling for the new menu design
Add and use variables for font size, font weight and light grey color

Add variables for three different font sizes as well as a variable for
the semi-bold font-weight and light grey color.
Set default font size to 16px and use new variables in menu.
Change header menu structure

Update styles to reflect the change to the menu structure:
header containing two menus. Also add some more spacing between
the two menus.