can i decree of strenght signature spells?

Yes — Decree of Strength can hit a signature spell, as long as that signature spell is a Mind card and it's in the opponent's hand.

Decree of Strength doesn't care about card type or the "Signature" supertype; it only checks domain ("you choose a Mind ([C]) card from it"). Multi-domain cards belong to each of their domains (Rule 134.1), and per the Vendetta patch clarification a [C] symbol on a multi-domain card is processed as any power of that card's own domains. So a Mind-containing signature spell in hand — Siphon Power, Final Spark, Fox-Fire, Icathian Rain, Guerilla Warfare, Danger Zone — is a legal choice and gets recycled. If the opponent's hand holds no Mind card at all, the instruction is simply ignored and nothing is recycled.

One note if you meant something else: Decree of Strength is itself just a Body spell with no Signature type, so it can't be run or used as a signature spell — Signature cards must carry the champion tag matching your Champion Legend (Rule 103.2.d.2).