Will Allah hold accountable those who treat their skin and buy creams and medicines out of extravagance, and is the use of skin-lightening creams considered altering Allah's creation?

Spending money on treatment is not extravagance unless it exceeds what is sufficient. Extravagance is spending money on what is permissible, but in excess of what is needed. Using creams and ointments to cleanse and improve the skin is not altering Allah’s creation, and there is no harm in it, provided it does not involve peeling or anything harmful.