Half Installed Kernel Issues

Apt and dpkg fails to purge and fix half installed kernels. First try linux-purge (opens in a new tab).

sudo linux-purge --fix

If it fails. Check the dpkg status of the packages:

dpkg --status linux-image-3.19.0-22-generic
dpkg --status linux-image-extra-3.19.0-22-generic

If the output states that the packages are in bad state, i.e. half installed or not fully installed, this means that they have broken apt-get and dpkg respectively.

The entries of the corrupted kernel packages must be deleted manually from the dpkg status file. This does not delete the files. It only ignores them.

# make backup of status file
sudo cp /var/lib/dpkg/status /var/lib/dpkg/status.backup
 
# delete ONLY references of the broken packages
nano /var/lib/dpkg/status
 
# finally
sudo apt-get update && sudo apt-get upgrade

Source (opens in a new tab)

Held back Upgrades

The most likely culprit is phased updates. Its a safety feature. Don't try to outsmart it.

Some users get the upgraded packages first, and have the ability to report broken package, instead of everybody getting a broken package at once and millions of users scratching their heads.

Kept-back packages due to Phased Updates will automatically resolve themselves, download, and install once the package is deemed stable in production usage.

Most users should DO NOTHING. It's not broken. Don't try to force upgrades. Just be patient and let the system work.

How to tell if Phased Updates is the culprit:

It's easy. Run apt-cache policy <package-name> on one of your held back packages. Look for the phased percentage. It's only present if the package is currently phasing.

apt-cache policy udev
...
 *** 1.20.3-0ubuntu1 500 (phased 40%) <------- There it is!
...

Flatpak

Flatpak runs all its applications in sandboxed containers. They don't have access to a user's home directory nor system files. This is made to ensure security.

This however means Flatpak apps don't have access to system GTK themes as well.

To force them to use Flatpak themes you first need a $HOME/.themes directory with your themes in it. If it doesn't exist that means either your current themes are at /usr/share/themes or /.local/share/themes. This is because flatpack blacklists certain system directories, which includes /usr/share/themes.

Once the themes are at ~/.themes, give all Flatpaks access to that specific directory and set their theme

sudo flatpak override --filesystem=$HOME/.themes
sudo flatpak override --env=GTK_THEME=<theme-name> 

In case of emergency this reverts all changes

sudo flatpak override --reset

Using Flatseal (opens in a new tab) is highly recommended for anything related to configuration changes with Flatpaks.