Lord Shiva, in the Padma Purana (Uttara Khand 236.19–21, 18) describes which Puranas belong to which mode:
"O beautiful one, the Vishnu Purana, the Narada Purana, the auspicious Bhagavata Purana, and the Garuda, Padma, and Varaha Puranas all belong to the mode of goodness. They are all considered auspicious. Know that the Brahmanda, Brahma-vaivarta, Markandeya, Bhavishya, Vamana, and Brahma Puranas belong to the mode of passion. And know that these six Puranas belong to the mode of ignorance: the Matsya, Kurma, Linga, Shiva, Skanda, and Agni Puranas."
As I understand it, Bramha-Vishnu-Mahesh are above all three guna, it is just that they are the presiding deities of a particular guna. Shiva presides over Tamas guna, this doesn't mean Shiva himself is Tamasic (duh) but he can help his devotees to conquer/destroy their tamasic qualities. Praying to destroy/conquer/defeat absolutely anything falls under tamas, even the tamasic qualities. Shiva helps here.
But how does this correlate to an entire puran dedicated to him being in the mode of ignorance (tamas)? Does it mean that reading/reciting this book helps to destroy one's negative qualities?
The very first Shiv Puran chapter seems like so, a man called Devaraj is destined to go to hell because he's an immoral person but just by chance happens to listen Shiv Puran from a Guru. When Yamraj's men arrive to take to him to Yamlok (hell) they are intercepted by Shiva's men hence Devaraj reaches Shivlok (heaven).
Bibek Debroy writes that this classification is tenuous.