Chiang Rai is generally considered a safe city, even at night. However, as anywhere else in the world, it is wise to take precautions. Stick to well-lit, populated areas, avoid less crowded areas, be aware of your surroundings and keep your valuables secure. Instances of violent crime are rare but petty theft such as pickpocketing can happen, especially in tourist-heavy areas. Local people are helpful and police presence is visible during the night. Use trusted transportation options during the night or use a taxi app if possible. As a solo female traveler, you should feel safe, but maintain regular traveler's vigilance.