When the public mood is a triple-strength cocktail of anxiety and alarm, there is something to be said for theater as comfort food. Heresy, I know — but it can be calming to sit for a while with a sturdily crafted, nicely cast play that was mild even when it was new. Case in point: William Nicholson’s anodyne biodrama “Shadowlands,” which traces the late-blooming 1950s romance between the author and Oxford don C. S. Lewis (his friends call him Jack) and the poet Joy Davidman.
