It's also written:
3.3) Capture an enemy capital. Capturing an enemy capital will raise the morale across all other provinces you own by 10%. (instantly at capturing)
In addition of that and if I remember correctly, morale drops by a maximum of 15% every day.. -- ..as long as there are enough enemy capitals, you can keep a high morale even without an own capital.
PS -- ..and also this:
3.2) Gold can be used to bring the morale up.