This tiny poem sets two things side by side: an absence and a sudden flash of color. The first couplet sounds flat and repetitive. "Dinner after dinner" suggests a routine of meals that drag on, and the missing wine feels like a small but nagging lack, as if the evenings are missing something that would warm them up.
Then the word "though" turns everything. The speaker sees a fox, and its fur is "red as sin." That phrase does double work. It describes the animal's vivid color, but it also brings in temptation and indulgence, the very things the dry dinners lack. The red the speaker can't pour into a glass shows up alive in the world outside.
The mood is quiet, a little wry, and gently consoling. The speaker doesn't complain loudly. They notice what is missing, then notice what is there instead, and the wild fox seems to make up for the wine.