Just to close the loop on this, I was able to figure it out, at least for Windows 7.
The short answer is that the actual image(s) can be changed using a tool such as "CustomizerGod," which, along with the Windows 7 prerelease task pane background, is included with this theme package (it's about the only thing from that collection that I like, and can be used independent of the actual theme).
The second piece is the text color, which cannot be changed by that application, and must be modified by a theme. Using UxTheme patcher, the stock Windows 7 theme can be modified like so:
Styles > ControlPanelStyle > Tasklink > TextColor : Color
font:font segoe ui 9 cleartype
textcolor:color solid white
>hot textcolor:color light green
pressed is same
disabled is dark gray
page is solid white
Finally, for anyone looking to make the caption bar opaque, (in solidarity with Windows Vista) below are the values that you'll want to change.
Stock/Old/Original:
MaxActive - opacity:int 80
colorizationcolor:int 0
colorizationopacity:int 100
textcolor:color solid black
glowcolor:color solid white
Active - opacity:int 80
colorizationcolor:int 0
colorizationopacity:int 100
glowcolor:color solid white
New/Fresh/Vista
MaxActive - opacity:int 0
colorizationcolor:int -16777216
colorizationopacity:int 100
textcolor:color solid white
glowcolor:color solid white
active - opacity:int 80
colorizationcolor:int 0
colorizationopacity:int 100
glowcolor:color solid white
If anyone's curious, I plucked those out of fediafedia's theme, if I remember correctly.