Since WordPress is licensed as GPLv2, you can license any addition (i.e. theme) with a license that’s compatible with version 2 of the GPL. Now here’s the unfortunate part … version 1 of the aGPL is not compatible with any version of the GPL. Version 3, however, is compatible with GPLv3 … but not GPLv2.
Short answer: no, you can’t license a WordPress theme with aGPL because neither version of the aGPL is compatible with WordPress’ license (GPLv2).
Link to license compatibility chart for reference …
Update
Strictly speaking, yes. WordPress themes are considered derivative works, so they must be licensed in a fashion compatible with the core project. WP is licensed as GPLv2+, so you can (if you want to) upgrade it to GPLv3 for distribution.
That’s important, because the only version of the aGPL that’s compatible with the GPL is version 3 … so your theme must be aGPLv3.
That said, you’re under no expectation to redistribute WordPress on your own … I’m just pointing out the compatibility here. But to keep things simple, I will always advise against using the aGPL. In practice, it doesn’t lend well for WordPress themes (feel free to ask me why, but that’s beyond the scope of this question).