The boxes are bitmap 100(1033) to 112(1033)
You should edit these bitmaps with Resbuilder.
Remember to use theme other than the one you are hacking.
The left panel background is not a bitmap but is gradient color.
Edit/change this using ResHacker.
Located in UIFILE 1.
On the right panel, find:
<style resid=taskpane>
element [id=atom(sectionlist)]
{
background: gradient(argb(0,192,192,192), argb(0,192,192,192), 1);
the numbers 0,192,192,192 represent the gradient color,
that is the color I use, yours might be difference.
192,192,192 is RGB color code
DO NOT change the 0 (alpha)
Once you done click compile script and save.
Remember to make a backup before you hack the file.