Update css.html. Fix sentence in css.html

BS3 commits: b298d2f481dca3f8f8c0c202ca1b9406a2c6849d,
fc03fac9e4fe95b29bac58b223234cd6135e3a88
"Thosing" [sic] => "Those"

BS3 commits: 2898e73ec49e66d529abb6a8483d5d9b09358b0f