Searching this forum, I saw a thread that says to delete the Kicad configuration folder, restart Kicad, and then in the dialog boxes, select “Use default footprints and symbols (recommended)”.
Notice the path substituion variable is KICAD7_SYMBOL_DIR, not KICAD_SYMBOL_DIR or worse still KISYSMOD. Also the symbol libraries should be in the new format .sym, not .lib.
So I think that you have not completely deleted the old configuration before rerunning KiCad for the first time, and some old v5 or v6 configuration remains.
I deleted the Kicad config folder (Ubuntu Linux) and then I purged Kicad before reinstalling.
Do you happen to know where is this config file that I have to delete?
The config directory is ~/.config/kicad and underneath it are 6.0 and 7.0. However there was no version directory in v5, it went straight into ~/.config/kicad. So you should remove everything in that directory.
You should not need to reinstall the packages. Purging does nothing for your personal configuration, that only affects system files. I also assume you have installed all the required packages as explained in the docs, as the symbols, footprints and 3D models are in separate packages. Also you should install the most up-to-date version, 7.0.6 as of writing.
The default color scheme for the bottom plane changed to blue from green, so this would be expected. You can switch to the old color scheme on preferences if you want (but I think blue looks better).
The color change is quite normal as Jonathan_Haas already mentioned. KiCad V7 uses different layer colors, and you can change the settings to the old colors if you like.
As for the Library Links…
Do you have the [Project]-cache.lib file with the project?
This is a quite essential part of older KiCad projects, especially to prevent problems like this. This file caches the schematic symbols, so the project can be “rescued” in case the original libraries are lost, and it is meant for exactly cases such as this. In KiCad V6 and onward, the schematic symbols are saved within the schematic file itself and problems like this should be a ghost of the past. PCB footprints are already saved in the [Project].KiCad-pcb file, and there should be no problem at all with PCB footprints.
Deleting the configuration directory fools KiCad into thinking it was freshly installed, and then it asks you to generate the library tables again. This does not help though to fix problems with an old project pointing to KiCad V5 libraries.
Your project can be fixed quite easily, under the preconception that old custom libraries are still present, and the old KiCad V5 schematic symbols can be replaced with KiCad V7 schematic symbols. You can update the schematic symbols with: Schematic Editor / Tools / Edit Symbol Library Links.
