steeldragons
Steeliest of the dragons
I would just define "dim light" as 5e defines it. No need in rewriting everything. 

Human: Three +1s and one skill proficiency
Haven't thought of that, this is a good addition!
Not sure if the number of +1s might need to be adjusted... If we assume the variant human is balanced (+1/+1/skill/feat) then maybe I can grant four +1s and a skill.
The variant human's "balance" is only concerned against the other normal races in the book. If you're going to remake the four basic races altogether, it doesn't matter how many ability score bumps you give.
They don't take over the standard races, they are an alternative. There can be a regular Elf and a 'light' Elf at the same table.