Use elementary's own keycap effect for shortcuts

[hanaral]

The ones used in the guide are images I think, but it shouldn't be too hard to either copy elementary's CSS or just edit them on to the image. Another thing, using images will make it more difficult to translate if elementary ever actually translates stuff like ctrl (ctrl on Apple's Turkish keyboards is kntrl)

[hezral]

i did consider using code for this at first, but opted not to since was focused on other things. will revisit this.