A theme can be as strict or liberal as you like. It's a guide, not the law. Whatever you choose can remain literal or abstract in your interpretation. The idea is to be about the work of writing, right?
It's interesting how many filters our creative steps go through as we make a thing into the world: a general theme, an abstract vision, environmental influences, our own mind, spirit, our physicality, our resources, drives, short and long-term goals, our arcing passions over the work, lack of motivations, frustrations, demands, etc. All of these and more give and take upon a final creative work. Why not guide ourselves on a single idea and see where it goes time and time again?
Blue: The sky is blue; I'm feeling blue; my blue shoes; the blue dishes; first blue thing I saw today; last blue thing I saw: blue things I hate/love; blue rhyming words; new words for the word blue; the history of blue; who came up with the word blue and what was happening in that very moment that required that word and what time of the world was it and what language was it; blue on blue; blue near blue; blue over blue; Rothko blues; singing the blues; blue spines of books lining the book shelf; blue items of clothing. On and on.
Now some of these are simple ideas on the theme of blue - until you think a little more on them, which is the point. Does the world need another poem about a blue sky? Who am I to say. Maybe it's not exactly about that, however. Maybe it's about going outside for a moment, changing up your routine, looking up to the sky and realizing there's absolutely no blue in the sky that afternoon. That's what you end up writing about. Maybe it's not a simple poem about blue shoes, but about why you haven't worn them in five years and can't bring yourself to let them go. That story belongs to your theme work. No one expects us to know the history of blue, but whatever comes to mind thinking on it makes a great flash fiction piece, I guarantee.
And then there's the color red...