When you are 1st level in D&D4 you get to choose some skills to be trained in. This gives you a +5 in those skills. As you move up in level all your skills are your appropriate modifier + 1/2 your level. So regardless a 30th level character has +15 minimum on every skill and +20 (15+5) on trained skills.
A background is a bit of role-playing that asks what were you before you were an adventurer? Most times it gives you a +2 in two (or sometimes just 1) skills. So maybe a fighter was once the apprentice of nere'do well Bard and spent his childhood stealing magical items. He would have a +2 to Arcana and +2 to Thievery
Themes are new from issue #399 of Dragon Magazine (seems odd to call it a magazine anymore) and they are more or less like kits were in AD&D2. You choose a theme and it gives you some pluses to skills and then at later levels you can choose different powers at different levels. not exactly a sub-class but more of a flavor.
To do these in the Basic Level model players need to have fairly good ideas of what their character was (or is since we are starting them younger) and where they want to take their character.
At Basic Level 1 they get their Background and maybe training in 1 skill. If the class has a "default" skill then it will be that one; ie Religion for Clerics, Arcana for Wizards and Thievery for Rogues. Clerics get their Channel Divinity power (Turn Undead), but no spells yet (just like Basic). Wizards get a spell.
At Basic Level 2 they get training in two more skills (or maybe just one, still looking at the options). Clerics get a spell as do wizards. Rogues and Fighters fight get one of their combat styles.
The idea here is to build up that list of trained and background skills and introduce the Themes to the character. In the end you want all the elements in place for that 1st level character.
While I consider the details here are the past posts that have lead me to this point.
- Character Life-Span development, this was more for modern games, but certainly good for D&D too.
- Immortal Level D&D. My first post on mixing D&D4 with D&D Basic.
- Playing D&D with Kids. Where I first mention my desire to play both D&D4 and Basic D&D.
Plus it gives me a good excuse to use all of these together.