The flag is effectively the Saudi flag, with its Muslim declaration of faith.
Underneath, the Arabic writing says "We pray to Allah that next year's hajj will be under the shadow of the Caliphate."
The end game for Islamists isn't a one-state solution in "Palestine" - that is merely a stage, and they are happy to recruit people who pretend to love democracy and human rights and the rule of law to get to that stage. Their goal - as stated explicitly in the Hamas charter - is to establish a single Muslim state in the entire Middle East.
What believing Muslim would publicly disagree?
(h/t Al Gharqad for translation)