>> I pick a new theme word every year and have done for… some time. This year's theme is consistency. <<

Go you!

>> It occurred to me as I said this that I use the secular/common/Gregorian calendar for this but there's no reason I shouldn't choose a new theme on Rosh Hashanah, or perhaps something specific to my religious practice, <<

If your religion has a set of virtues or other values, that would make a good match. Tikkun olam would work great.

>> or even every month…? <<

A virtue of the month works well, but you could also choose a personal aspiration or whatever.