Learn Croatian: Sve smjernice su u skladu s pravilnikom. - All guidelines are in accordance with the rulebook.

A complex preposition is simply a multi-word phrase that functions together as a single preposition. The phrase u skladu s translates to "in harmony with" or "in accordance with" and acts as a single, fixed unit to connect the guidelines to the rulebook.

Which case does u skladu s trigger for the noun following it?

It triggers the instrumental case. Because the phrase ends with the simple preposition s (with) denoting association or alignment, the masculine noun pravilnik (rulebook) takes the instrumental suffix to become pravilnikom.

Could I just use prema to express conformity here?

Yes, you could say prema pravilniku (according to the rulebook). However, u skladu s is preferred in formal contexts because it emphasizes strict compliance or being "in line with" regulations. Remember that swapping them changes the grammar: prema requires the dative case, while u skladu s requires the instrumental.

Just a quick refresher: why do we use sve instead of svi for "all"?

The noun smjernica (guideline) is feminine. In the plural, the nominative form is smjernice. The pronoun for "all" must agree in gender, number, and case, so we use the feminine plural form sve. You would use svi for masculine plural nouns.