Are the "twenty" themes compatible? Can you update 2011 to 2014?

Each TwentyXX theme can be seen as a separate, independent theme, just like any other theme, so you can switch back & forth between any of them.

so you can switch back & forth between any of them

– well I know WordPress advertises that this can be done, but in most cases each site’s CSS is so customized that I wouldn’t even try it. That’s why I was asking.

True – CSS is theme (and site) specific – but any custom CSS will not be applied to another theme. It’s saved on a per-theme basis.

But the twenty-something themes are completely different and separate from each other. Switching themes isn’t updating in a code sense. Each of the default themes is updated as WP core is changed – and they are updated when you update WP. At some point, the WP developers may decide to “retire” an old default theme – but twentyeleven is still being updated – most recent was 9-3-2014.

That said, the newer default themes may give you a more “updated” look and newer functionality in a more general sense – as they are coded to the newer “cool stuff” that all websites can do.

but any custom CSS will not be applied to another theme. It’s saved on a per-theme basis.

In cases where the theme has its own Custom CSS option, yes, that’s true (none of the TwentyXX themes have a Custom CSS option, though). It’s also true if you create a child theme; the CSS from the child theme’s stylesheet won’t get passed along to another theme.

However, the rules from CSS plugins (like Custom CSS Manager) will be in effect across multiple themes. So a particular rule that was written for one theme may have an effect on the elements of another theme, depending upon the selector that’s used, especially if the selectors are created using only common WordPress classes, like hentry or entry-title.

Thanks. So basically the answer is to treat them as separate themes and not upgradable from one to the next one.

Yes. You can actually start with TwentyFourteen and switch to TwentyTwelve if the appearance of that theme suits your needs better.

I was asking because a freelancer built a site for us in twentyeleven and I wondered if I could just change to to twentyfourteen (figuring the later year would be better coding) but it looks like the answer is no – it probably won’t look the same and it has to look exactly the same.

That is correct, there is no resemblance between one TwentyXX theme and another, either in terms of functionality or appearance. In fact, each is quite different. The names just signify the year in which the theme was released.

Thanks everyone, that’s the answer then.