I used to do that in my Shul in the Social Hall for Eicha, but always a balance trying to make it much darker than normal but bright enough for people to read. But in the main Sanctuary none of the lights are on dimmers and only a few large groups of lights so no easy way to "dim" and we just left all the lights on. I'm pretty sure (but don't have a source, so just a comment) that this is a Minhag to just increase the sense of mourning but not strict in any way. Plus, just like someone with a

bad back doesn't have to sit on the floor, someone who can't see well can use regular light. In any case, this year it was Maariv in Shul and Eicha on Zoom, so not an issue.

The Shulchan Arukh says ואין מדליקין נרות בלילה, כי אם נר האחד, לומר לאורו קינות ואיכה Doesn't say anything about where you should sit