If you ever get cold...
If you ever get cold, just stand in the corner of a room for a while. They're normally around 90 degrees.
This joke is a combination math/pun joke, with a little physics thrown in.
The crux of the joke is that we tend to build houses with rooms that have adjacent walls that are orthogonal to each other, and so the angles of the corners tend to be 90 degrees.
Degrees are both used to measure angles and temperatures, however, so the pun is that the jokester is assuming that both are interchangeable.
90 degrees Fahrenheit is quite warm to be in for extended periods of time, and 90 degrees Celsius is almost boiling.
In Kelvin, the joke makes no sense, so we have to assume they weren't considering it.