How accessible are CMS default themes?
In short and in plain language
This varies a lot. Do not rely on a theme being called accessible.
Good contrast, focus or keyboard use are often missing. Your own changes can also add faults. What counts is your live site.
An audit shows what your theme needs. Or whether a switch is better.
This varies widely, and you should not rely on the claim that a theme is accessible across the board. Even well-meaning themes frequently show basic defects in practice: missing landmark structures, insufficient color contrast, mega menus that cannot be operated by keyboard, poorly marked-up sliders or missing focus indicators.
On top of that, conformance depends on the overall picture: even a solid base theme can acquire barriers through your own customizations, page-builder elements or embedded modules. The delivered state of your live site is decisive, not the theme's demo state.
An audit shows precisely which adjustments your specific theme needs, and whether these can be implemented in a targeted way or a switch is more sensible. More on this under accessible web development.