Comment 1 for bug 738293

As for the folders,you can hide folders by right-clicking on them and clicking on "Hide folder" in the popup menu. There's no way to show hidden folders though,which has to be implemented in the future.
Also,if there are new emails,the inbox becomes bold,so there is no need to bolden subfolders and labels as well (postler boldened them for a while,but this has been changed a couple weeks ago or so).

As for the marking, "unread" marking is quite straight forward,you just have to click on the symbol and it gets marked.
Moreover,i think you confuse "unread" and "new" mails: unread mails are messages that haven't been opened and read yet,so their number appears next to the folder; new messages are emails that the user hasn't "seen" yet,so as soon as you open a folder containing new messages,you see that there are new ones and they aren't new anymore but only unread.