I suggested changing this sentence to "Vi estas mia lumeco en la mallumeco." for a very simple reason, the light and darkness here are abstract, not concrete.
For the record, "-ec-" changes some thing concrete into something abstract.
-ec does not change concrete into abstract. It denotes a quality or a state. "Lumeco" means "luminosity".
"Lumo" is the proper root meaning "light." I see no need for the abstract affixe at all.
The light from the sun is composed of photons, something concrete and real that you can interact with physically, it literally touches your eyes an allows you to see.
The "light of the heart" however exists only in your mind, it can not touch you the way that sun light can if you're outside for too long in summer.
That's the difference between concrete and abstract, that's why the abstract affix "-ec-" exists.
It is better to express and communicate with concrete vocabulary. Duo is doing great promoting Esperanto, let's support Duo.
- It is better to express and communicate with concrete vocabulary.
That's an interesting statement. How so?
He's so good that it's difficult to get through the other courses which have text-to-speech.