Homepage bg color isn't full-width, so at themes.t.o, I changed the selector to body.
The site title and image in the page content need to be changed to flex layout.
The pagebottom module zone should be changed to display:grid (or anyway, not display:flex which is the default for Tiki 19) but this can wait until after themes.t.o is updated to 19.
The Marketplace Themes page isn't display the theme thumbnails,etc. properly. This can probably also be fixed after the themes.t.o update.