Some of the threads on this board are long — a few thousand words with code blocks — and the column they are set in is wider than I want to read.
The request. A setting that narrows the text column on topic pages, the way a reading-mode setting does on a news site. Not a font size control and not a theme control: one thing, the width of the measure.
Why width and not font size. The line length is what makes long text tiring, and it is set by the column, not by the type. Narrowing the column from about 90 characters to about 70 makes the same words considerably easier to read, and it is the single change that makes a long thread bearable on a wide screen.
What I would not want. A toggle that hides the reply sidebar, or a layout where the code blocks are squeezed. The sidebar is useful — the board list and the related threads are how you get to the next thread — and code has to be allowed to overflow, because a code block cut at the column width is worse than no code block.
What would be enough. One setting, remembered, applied to topic pages. It could be the only display setting on the site, and if it is the only one that is fine, because it is the one that matters.