Make backups of your .map before adjusting skybox kvp. Bad values can crash radiant.
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Sky textures emit light. Pick a dark one to start. Change the sky texture in radiant, change the worldspawn skybox kvp. Add xmodel,skybox to your zone_source/mapname_patch.csv if you have one.
Reduce "sunLight" kvp
Reduce "sunRadiosity" kvp
Use lower intensity values than 1 on lights. Dont use any "primary omni" lights.
Use caulk shadow.
You could also make a black skybox and add the stars or cloud meshes from another skybox.