Content styling

[NullDivision]

It would be nice to have a way to modify the .selectable-content. As is, I can't add iron-flex-layout to the .selectable-content and instead have to apply it to the items.

[josiah47]

I also needed this for flex layout, I ended up adding this:
.selectable-content {@apply(--paper-menu-selectable-content);}
to paper-menu-shared-styles-html
then was able to:
--paper-menu-selectable-content: { @apply(--layout-vertical); @apply(--layout-flex); };