Legal Question in Employment Law in California

"Sexist" is not a legal term or concept. The law prohibits improper treatment because of sexual orientation but dress codes and the like can be reasonable work place requirements. certainly an employer could bar nails painted with offensive language on them; perhaps the employer can ban male employees from wearing nail polish. The problem is that is it worth risking your job to fight for a right you likely do not have? If it is a big company with a separate HR division, you can try going there to see if they will back you, but in a small company you will have to deal directly with the person who already told you that you can not wear nail polish.

Dress codes can be enforced to ensure neat, clean, modest, propriety, professional, non-controversial, etc. There is nothing wrong with employers demanding appearance that does not bother customers. Legally they can ban visible piercing, tattoos, extreme make up, nails, hair styles, cloths, colors, etc. If a prospective or current employee doesn't like the dress code, they shouldn't be working there.