I did some experimentation to get DPI changes working on 3.4 (I’m on the very latest 3.4.0.24), and I’ve discovered a way to make it work since dconf write no longer seems to work, even with the vendor locks disabled:
First, make sure you have the sailfish graphics packages installed:
devel-su pkcon install --allow-reinstall -y sailfish-content-graphics-default-z1.0-base sailfish-content-graphics-default-z1.25-base sailfish-content-graphics-default-z1.5-base sailfish-content-graphics-default-z1.75-base sailfish-content-graphics-default-z2.0-base sailfish-content-graphics-closed-z1.0 sailfish-content-graphics-closed-z1.25 sailfish-content-graphics-closed-z1.5 sailfish-content-graphics-closed-z1.75 sailfish-content-graphics-closed-z2.0
Make your edits, for example, I edited /etc/dconf/db/vendor.d/silica-configs.txt and set it to the following:
[desktop/sailfish/silica]
theme_pixel_ratio=1.25
theme_icon_subdir='z1.25'
After saving, I used dconf compile to recompile the keys directory into a binary, like so:
dconf compile /etc/dconf/db/vendor.new /etc/dconf/db/vendor.d
Then, backup the old vendor file:
devel-su
cd /etc/dconf/db
mv vendor{,.bak}
Then move your new vendor file into place:
mv vendor.new vendor
After rebooting, your DPI will be set, and your icons will show up since you have the graphics packages installed from earlier.
I just tested it on two XA2 Ultras, one fresh flashed, and one OTA’ed.
I hope this helps someone, at least. Let me know if you run into any issues.