Acne has been proven to be caused by these foods:
Milk and Dairy Products - Systematic evaluation carried out in 2018 has shown that the individuals between the age of 7 and 30 that consume any sort of dairy products are more inclined to having acne than those who do not consume dairy products (Juhl, 2018).
Dark Chocolate - A study, which included 25 male subjects with acne-prone skin, has shown that the condition of the skin has worsened in 4 weeks due to daily consuming of 25 grams of dark chocolate (Vongraviopap, 2016).
Sugary drinks - A recent study confirmed that daily indulging of sugary drinks highers the chance of having bad skin in teenagers especially if the daily amount of sugar gained from sugary drinks exceeds 100 grams (Huang, 2019).
Foods with a high glycemic index - as previously said dairy products, dark chocolate and sugary drinks cause acne, so we can conclude that foods which have a high glycemic index may contribute to not having clear skin.
It has to be mentioned that eliminating those foods from your diet only solves part of the problem and it does not mean that acne will go away completely but at least elimination of those foods may help in your skin not worsening.
Sources:
Huang, X. 2019. Daily Intake of Soft Drinks and Moderate-to-Severe Acne Vulgaris in Chinese Adolescents. The Journal of Pediatrics. 204. 256-262. Accessed from: https://www.jpeds.com/article/S0022-3476(18)31208-3/pdf (26.1.2019).
Juhl, C. R. 2018. Dairy Intake and Acne Vulgaris: A Systematic Review and Meta-Analysis of 78,529 Children, Adolescents, and Young Adults. Nutrients. 10 (8). Accessed from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6115795/ (26.1.2019)
Vongraviopap, S. 2015. Dark Chocolate Exacerbates Acne. International Journal of Dermatology. 55 (5). Accessed from: https://onlinelibrary.wiley.com/doi/abs/10.1111/ijd.13188 (26.1.2019)