As described in the 13th Age core book Backgrounds are very personalized and unique and flavorful. While I like what they intended with all that, I see a lot of pitfalls in letting players just come up with anything as a background, especially in a group made of complete strangers online. So, I'm using the list of suggestions out of the core book as an actual list of Backgrounds, with the caveat that if you want a skillset that isn't present you can discuss it with me and we'll likely add it in.
Also, in the interest of rewarding player creativity you have the option to re-flavor any/all of the backgrounds you pick from that list.
For example, Tim and Becky are both making characters. Tim a rogue, Becky a bard. Tim's rogue is something of a second story man, not afraid to do a little parkour action getting from one building to the open window of another. Becky's bard is a street performer from Santa Cora, so not only does she make her way through life tumbling for an audience, she has done so in a city with a monotheistic church that reaches all the way up into the clouds.
Tim might be satisfied with just putting down Acrobat as a background and calling it good. Sure, he'll have Burglar, and maybe a third as well, but we're just going to focus on Acrobat at the moment. This way it would cover various situations like swinging from a sign post to a window seal, or tightroping from one building to another.
Becky on the other hand puts down Santa Coran Street Tumbler. Same basic idea, she can pull off some gymnastic tricks, but she also knows a little about those who worship the gods of light.
Now, you might be thinking Tim is getting the short end of the stick here, and you're not far off. For one, I would like to eventually see all the Backgrounds on everyone's sheet get a little bit of flavor. No, you don't have to start off with flavored up Backgrounds, and no it won't cost you any experience points or anything like that. When you feel inspiration hit, let me know and I'll go over it with you, and we're good.
For two, Becky's put just a little more thought into her Background than Tim, and I think that warrants something of a reward. She took the time to read a little about the setting, and possibly even added to it (I don't think I've read anything about Santa Cora being known for its street performers, but now we have the possibility), so yes, she gets to do a little more with her Background than if she'd left it at Acrobat.