Windows 10 Compatibility

although my main sidebar doesn't appear to be skinnable any more

Yes, you are right. There are problems with default skin.

To select skin for the standalone TLB, right-click the empty space of toolbar and select Toolbar Properties. On the Skins page you can select the skin.

I've found that I can make it pleasantly transparent by right-clicking empty space, TLB Settings, Toolbar, Appearance, and then turning on transparency (and setting to a value of 1% or more). Looks nicer than plain white until it's fixed.