Fix light theme name
Fix light theme name

Signed-off-by: Yukai Huang <214563a1d5868d6fb504f80fe7b386709fe7e6ae@gmail.com>

hackmdio/hackmd,jackycute/HackMD,hackmdio/hackmd,hackmdio/hackmd,jackycute/HackMD,jackycute/HackMD