In terms of making the blog meet the current set of accessibility standards.
The number of themes available that meet those standards (and that are also mobile friendly) is minuscule. And of the ones that do exist, they are not really what I want, because they are mostly about the images, and I am (mostly) about the words.
At some point in the future – I don’t expect it will be soon – the number of accessible themes will grow. Of course given the lack of focus on accessibility I doubt that it will be soon.