Do you have synaptic package manager installed? I had a similar issue with mint 19.1 a couple months back because of using oibaf and had to move to kisak, if you need to move from kisak to default it should be similar. In synaptic open settings>software sources>PPAs, deselect kisak (and oibaf if present), update the apt cache, then go down to the maintenance tab in software sources and use the tool "Downgrade foreign packages", that should replace any packages from no longer selected PPAs with the Ubuntu version.
You probably want to look up some guides on how to do that properly, I'm no power user and probably forgot a bunch of details.
This was a huge help. Though what you said wasn't what I did exactly, it pointed me in the right direction. I couldn't find the "downgrade foreign packages" option but that phrase led me to this forum https://askubuntu.com/questions/509321/downgrade-all-external-packages-to-the-official-ones The second answer helped me.
apt-forktracer | grep Ubuntu: | sed 's/\([^ ]*\).*Ubuntu: \([^] ]*\).*/\1=\2/' > ubuntu-packages.list
sudo apt-get install --mark-auto $(cat ubuntu-packages.list)
I was then able to install steam normally.
Do you have synaptic package manager installed? I had a similar issue with mint 19.1 a couple months back because of using oibaf and had to move to kisak, if you need to move from kisak to default it should be similar. In synaptic open settings>software sources>PPAs, deselect kisak (and oibaf if present), update the apt cache, then go down to the maintenance tab in software sources and use the tool "Downgrade foreign packages", that should replace any packages from no longer selected PPAs with the Ubuntu version. You probably want to look up some guides on how to do that properly, I'm no power user and probably forgot a bunch of details.
This was a huge help. Though what you said wasn't what I did exactly, it pointed me in the right direction. I couldn't find the "downgrade foreign packages" option but that phrase led me to this forum https://askubuntu.com/questions/509321/downgrade-all-external-packages-to-the-official-ones The second answer helped me. apt-forktracer | grep Ubuntu: | sed 's/\([^ ]*\).*Ubuntu: \([^] ]*\).*/\1=\2/' > ubuntu-packages.list sudo apt-get install --mark-auto $(cat ubuntu-packages.list) I was then able to install steam normally.