Yes and no. I just corrected a minor bug to ensure that they are stylable (thanx for pointint it out!) - BUT the horizontal sidebar divider is still not because it is actually a gradient and set very differently from the other lines. Replacing the gradient with a plain color gives an ugly result and my attempts to have the gradient fetch a customized color did not succeed.
However, you can of course style the concerned parts the usual way. Do a search for "linear-gradient" inside tiddler $:/themes/tiddlywiki/snowwhite/base . You'll need to change the r,g,b values inside the rgba(r,g,b,a) to something between 0 and 255, e.g rgba(255,0,0,.5) for r (i.e red)
<:-)