Is it permissible to use clear eyebrow gel to groom and set thick eyebrows? And is using sunscreen that slightly lightens skin tone for protection considered tabarruj (displaying one's adornments)?

The default rule regarding a woman's adornment is permissibility, unless a religious text explicitly prohibits it. There is no text that prevents a woman from using moisturizers or oils on her eyebrows, combing them, or using something that temporarily lightens skin tone and protects from the sun.

However, a woman must avoid displaying this adornment to non-mahram men. The Muslim woman is required to cover herself and not display her adornment to anyone other than her husband and mahram relatives, in accordance with the Almighty's saying: "And not display their adornment except what is apparent thereof, and let them draw their headcovers over their bosoms, and not display their adornment except to their husbands..." (An-Nur: 31).

What is meant by the Almighty's saying: "except what is apparent thereof" is the outer garments. The preferred opinion is that it is not permissible to display the face and hands to non-mahram men.