Learn Finnish: Kampaaja vaalentaa hiukset, joskin se voi huonontaa niiden laatua. - The hairdresser lightens the hair, although it can worsen their quality.

Both verbs use the transitivizing suffix -ntaa / -ntää to turn an adjective into an active verb that means "to make [adjective]." Here, vaalea (light/blonde) becomes vaalentaa (to make lighter), and huono (bad) becomes huonontaa (to make worse).

Why is the object of the first clause (hiukset) in the nominative plural, but the object of the second clause (laatua) in the partitive?

Hiukset takes the nominative plural (the standard total object form for plurals) because the lightening is viewed as a completed, intentional action done to the whole head of hair. Laatua is partitive because "quality" is an abstract, uncountable concept, and the worsening is an indefinite change in degree.

What is the exact meaning of joskin, and is it different from vaikka?

Joskin means "although" or "albeit." It functions very similarly to vaikka (even though) but is slightly more formal. It is typically used to introduce a contrasting fact, side note, or concession attached to the main statement.

What exactly does the pronoun se refer to in the second clause?