Ok so I just bought the windhelm house after doing the stormcloak questline. I did nothing else in windhelm (aside from rob everyone blind without being noticed). However when I went back to the steward with 10000 gold to spend, the option to buy decorations was not present. Am I supposed to do the murder investigation? Is there something else I'm missing? Or is this another bug?
According to the Elder Scrolls Wiki:
Talking to the Jarl's Steward, Jorleif, enables the purchase of decorations for Hjerim; however, the quest "Blood on the Ice" must be completed before buying decorations and the cleanup package from Jorleif, or else glitches ensue.
In other words, you need to finish the murder investigation before you can purchase decorations.