The same place the culture of any other province comes from - Years of "isolation"
Then it seems they didn't just come from nowhere afterall.
The center of the Arena ended up becoming the "Eye of the Storm" - a place of calm surrounded in blood. Displaced people from all eight corners of Tamriel found themselves having to band together for survival, creating a new, distinct cosmopolitan culture in the process. It then became center of the Empire because of the strong racial blend - They made perfect ambassadors, and the "colorblind" culture allowed the army to work together smoothly, while bringing advanced Mixed Unit Tactics by tapping into the natural abilities of the component sub-races of the Cyrods.
You realize that's precisely what our modern Cyrodils are: a fusion of Nedic, Nordic, Ayleidoon Merish, Kothri, and Akaviri peoples. Like I said, the good bones are there.
It needs the culture and race. But it doesn't need to be one defined by skin skin color/texture or lifespans of its component members. Cyrods had eight subraces before Redguard
Cyrodils aren't one color/texture. You're stereotyping them as all being swarthy Nibenese. Colovians, with their decidedly more Nordic blood, are quite pale.