Open your console and type

sv_cheats 1
sv_skyname ***

Replace *** with a sky name from the listing below.

Hint: This works only on a private or local server.

cs_baggage_skybox_
cs_tibet
embassy
italy
jungle
office
sky_cs15_daylight01_hdr
sky_cs15_daylight02_hdr
sky_day02_05
nukeblank
dustblank
sky_venice
sky_cs15_daylight03_hdr
sky_cs15_daylight04_hdr
sky_csgo_cloudy01
sky_csgo_night02
sky_csgo_night02b
vertigo
vertigoblue_hdr
sky_dust
vietnam