bm-overlay class does not get applied to body properly

[stevenkuipers]

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I ran into an issue where my font changed when I would open and close the menu. When the bm-overlay class is added to the body it is a string concatenation, but there is no space in front of bm-overlay so it would concat with the last class I had on body. Using classList instead of className is a beter way to add and remove classes.