First, congratulations on the creation of these materials and the use of a pleasantly consistent "branding." I'm becoming quite fond of the LibreOffice document symbol and the color scheme.
I agree with Jean-Francois that the black border and large black triangle should not be so emphatic or heavy. Part of the problem is that this sort of thing doesn't scale. As we go larger, we need to go finer. I also don't think there is any need to bleed to the edge, if that is what was done. A modest line (maybe 3mm) and smaller corner cut are appropriate. (And consider making the larger cut corner not solid or else much lighter.) Also, going to a greyed (lighter) tone is helpful, as is done on the LibreOffice splash/main display. I also agree with Tom that the border and the corner cut could be the same colors as used for the different document types.
Finally, if the border just doesn't work at that scale (because of corner-cut proportions), maybe something more like the PDF cover page is needed, but using the theme and non-solid colors instead of the black symbol that is there now.
- Dennis E. Hamilton
Individual Member, OASIS ODF TC
[Just butting in because I like what you are doing.]