I'm on Trusty and would like to continue installing the newest mainline kernels. These started to depend on linux-base >= 4.5, while Trusty only has linux-base 3.5 in its PPAs. Is it safe to get linux-base from a xenial PPA and install it on my trusty? Could it break something?
It's safe to download linux-base 4.5 from the official Ubuntu Download Page for linux-base_4.5ubuntu1~16.04.1_all.deb and double-click the linux-base_4.5ubuntu1~16.04.1_all.deb that you downloaded to open it for installation in the Ubuntu Software Center. There won't be any dependency problems either.