Introduction drawer has rounded corner on bottom left side

The following discussion from !105 (merged) should be addressed:

• @sebkur started a discussion: (+7 comments)

  …although I changed the scrim color to white on the dark theme because the standard Surface background color (black for the dark theme) doesn't have any contrast to the rest of the UI if it is overlayed with a transparent black layer. Black looked good for the light theme though, so I tried using inverted colors for the dark theme.