style(theme): style dropdowns, DRY some effects
style(theme): style dropdowns, DRY some effects

Style dropdown content: the lists underneath the dropdown section now
line up better, have a background, and have a shadow

DRY common visual styles: border-radius and two sizes of shadows are now
defined as variables for consistency's sake

0u812/nteract,nteract/composition,jdfreder/nteract,jdfreder/nteract,temogen/nteract,rgbkrk/nteract,captainsafia/nteract,jdfreder/nteract,captainsafia/nteract,nteract/nteract,rgbkrk/nteract,nteract/composition,nteract/composition,0u812/nteract,0u812/nteract,jdetle/nteract,nteract/nteract,0u812/nteract,captainsafia/nteract,nteract/nteract,rgbkrk/nteract,temogen/nteract,captainsafia/nteract,jdetle/nteract,jdfreder/nteract,rgbkrk/nteract,rgbkrk/nteract,temogen/nteract,jdetle/nteract,jdetle/nteract,nteract/nteract,nteract/nteract