It really depends and there is no rule. Some years it's raining nonstop for 2 weeks, some years the weather is almost as if it is summer. But in general, the temperatures are mostly pleasant, not too cold and not too warm, and it can be rainy but nothing extreme. It also depends on the town you visit. Opatija, for example, which we could also say is on the Istrian peninsula tends to be rainy because of its location under the Učka mountains (don't know the exact reason but something due to its location) while the central and west part of Istria (including
Rovinj) doesn't have that issue and the rain is not as common.