Ok, that's super helpful on both counts! I totally forgot that there's an option to create a theme layer as well as a layout layer, and that's EXACTLY what you need to do. In your advanced customisation area, create a style-specific layer and base it on Minimalism (it should be right down the bottom). That raw source you found is lj's current version, which is why there's so many errors. What you actually need is a version from a few years ago, which I've tracked down for you and uploaded here. Edit the style-specific layer, paste that in and compile as usual - you shouldn't get an error, I took the lj-specific stuff out already, but do poke me if you do. ^_^

Once you've done that, your developer area should have Minimalism and Minimalism Penguins both listed. Go back to the normal customisation area, select Penguins as your theme (it should be listed in "Your Custom Layers" in the side bar) and uncheck both boxes in the custom css area. Then just paste in the css from Minty Apple and you should have it all showing up properly. :D I've tested it on an rp journal of my own and both the title and that diagonal stripe show up. The only thing is, it's an empty journal for now, so it doesn't have entries to show me whether they align the way they should - I'll cross my fingers that all that stuff is ok!