Learn Swedish: Djärv var kollektionen som designern hade visat före festen. - Bold was the collection that the designer had shown before the party.

This is called stylistic fronting. By putting the predicative adjective (Djärv) at the very beginning, it creates a dramatic or narrative effect, emphasizing exactly how bold the collection was. Because Swedish strictly follows the V2 (verb-second) rule, the verb var must come immediately after the fronted adjective, which pushes the subject kollektionen to the third position.

Does the adjective Djärv still need to agree with the noun even when it is moved to the front of the sentence?

Yes, absolutely. Even though Djärv is placed at the beginning and separated from its noun, it still describes kollektionen (an en-word in the singular). If the subject were an ett-word, like beslutet (the decision), the adjective would have to take a t-ending to match and become Djärvt.

Why is the definite form of designer spelled designern instead of designeren?

For many nouns ending in -er that refer to people (including English loanwords like designer or partner), Swedish simply adds an -n to form the definite singular. Adding the full -en ending would sound clumsy and be hard to pronounce, so the extra vowel is dropped.

Could I use innan instead of in this sentence?