I'm making a new character, an Elf who uses Light Armour, Archery, Magic and one-handed. He's going to join the Thieves Guild and play a role in the Civil War.

He's going to be a Dull Golden (Or a good but not perfect character) and I'm not sure which race he should be...

This question is one of personal preference. If you want to be more of an archer go with the Bosmer. If more of a mage go with the Altmer. The Bosmer starts off with better thief skills, but in the long run it doesn't matter. The Altmer's special power and racial ability will always be good for a mage, the Bosmer's is very limited and doesn't apply to any class. If an Altmer, your character may have left the Summerset Shadows.

A Bosmer would be more interesting to me, personally. You would endanger yourself if you choose to freely travel within Skyrim's borders as a snooty High Elf.

Light armor and archery then straight away I'd think of Bosmer. Especially if you are joining the thieves guild so would also be relying on sneak etc

The skillset is more or less what the Altmer soldier's I've encountered have used. But I'd suggest Bosmer, Like the Khajiit they make natural thieves.

Ok thanks, made him now and he look pretty cool

Which race did you settle on - Altmer or Bosmer?

Bosmer

I wouldn't on Oblivion though, they look like girls dolls in Oblivion :/ But Skyrim's look rather nice