change bg
change bg

Former-commit-id: c56103cce06fe1ac0992f42816e5b2ffcb9f4abc [formerly 6e21511f8e4ab1217f3b5126123c4337f5d0b6c9]
Former-commit-id: e5e3d70cedd3d6d7c3b5b61e0d38a8bc903e963d
LibraryCarpentry/librarycarpentry.github.io,LibraryCarpentry/librarycarpentry.github.io,LibraryCarpentry/librarycarpentry.github.io