Question: Is there any way to customize the border color of a `Tkinter/ttk` widgets or frames?

Answer: 
Yes, you can create your own *style* and apply it to the widget or configure the attributes of an existing style. 

Some documentation and examples are in the official docs, specifically the section about [ttk.Style](https://docs.python.org/3/library/tkinter.ttk.html?highlight=ttk%20style#tkinter.ttk.Style).

Additional information on using styles (for several languages) can be found at [tkdocs.com](http://www.tkdocs.com/tutorial/styles.html).
Regarding a border on ttk.Frames: in lieu of styles, the best I can think of is to insert a ttk.Frame within a ttk.Frame. The outer frame would use a standard "background" property with the border color of choice, and a padding equal to the desired border thickness. The inner frame then obscures the middle, so you have a line only around the edges. Works, but seems a little bit hacky (and a bit less wieldy if you're creating many widgets/borders).

I've searched quite a bit and haven't found or figured out a way to use a style to change a ttk.Frame's border color. Using something like `style.configure( 'SeletectedItem.TFrame', highlightcolor='#78F', highlightthickness=2 )` or `style.configure( 'SeletectedItem.TFrame', highlightbackground='#78F', highlightcolor='#78F', highlightthickness=2, borderwidth=2 )` (config options that work on a regular Tk.Frame) doesn't seem to do anything. With `style.layout()` I find just `[('Frame.border', {'sticky': 'nswe'})]`, and from `style.element_options('TFrame.border'`) there is just `('-relief',)`. I don't know if I'm using the wrong option names or syntax (or what options are available for `.configure()` for different widget classes), or if this just isn't possible with styles. I'm interested if anyone has extra info or knows if this can be done.