Chapter 31 of White Yajurveda contains the Purusha Suktam which tells us about the Supreme Nature of God. I have read that Lord Aditya is the deity of the Purusha Suktam. So does it means The Supreme Being described in the 31st chapter of the Yajurveda is Lord Surya?
No , the Purusha Sukta of Shukla Yajurveda is not fully dedicated to Aditya.
We can say that because Surya or Aditya itself is said to be born from the eyes this Purusha in Yajurveda Samhita 31.12 . The devata of Shukla Yajur Veda Purusha Suktam Mantras from 1-16 is Purusha AdiBeej पुरुष आदिबीज - ( Purusha whom is the primary seed of creation).
Surya or Aditya is devata of Mantras from 16-22 ,So only those mantras are dedicated to Surya or Aditya.
चन्द्रमा मनसो जातच्श्रक्षो: सूर्यो अजायत |
क्षोत्राद्वायुच्श्र प्राणस्य मुखादग्निराजायत ||12||
12 The Moon was gendered from his mind, and from his eye the Sun had birth;
Here is Hindi Translation , We can see Rishi And Devata of the sukta here.
And here is English Translation