Select the AddOn you want, click "tree", then download the individual files. They will need to placed in a new folder with the AddOn's name. It's the same idea as manually downloading from a SVN rep (aka not using a external program or having prebuilt zips)
Too many prominent borders make your UI look cluttered, and distract from the parts that actually provide information.
I disagree in this specific case. While it's true that borders and dividers are a huge source of contention in UI usability studies (i.e., some studies show the same use of dividers and borders help, while other studies show an identical configuration hinders), in this case, I would say "they don't hurt."
There are very few usability studies performed in settings where the background of a GUI consists of a moving, 3D modelled image. (More studies have been done when the background setting is a video, but the GUIs in those studies didn't model complex interactions.)
Strong horizontal and vertical lines with high background contrast are important as visual cues for your eye to locate both interactible and informational controls within (or upon) a 3D field. The stronger the lines, the greater the linearity, and the higher the contrast, the fewer saccades your eye must perform in order to fixate on an element. If the lines are less linear (i.e., curved) or have lower background contrast, they need to be proportionally stronger (thicker) to achieve the same ease of identification and recognition. (This is part of the reason why Blizzard's default UI performs poorly in stereoscopic rendering environments.)
Perhaps some differentiation between informational and interactible controls in their borders and dividers would be helpful in this example, but I think it's a good start.