Older plugins typically used a static bitmap, which made it easy to change background colors and labels. V-Vocal is one that was easily customizable. When the VST SDK came along it provided a standard method for graphics but it results in hard-coded UIs that can't be modified. Newer plugins tend to use more sophisticated tools such as JUCE, which paint the controls at runtime so end-users can't modify them. A few vendors (Blue Cat, Meldaproduction) let you change visual elements but that capability has to be coded in.
The reason graphics are so unmutable is that it's a challenge for developers to create UIs that are compatible with both Mac and Windows platforms, which each treat graphics very differently. That's one of the reasons JUCE is so popular: it's cross-platform so the programmer needn't worry about whether the knobs will paint correctly on both. Unfortunately, it also means a lot of extra work on the programmer's part to create tools to let users alter the plugins' appearance. The ability to customize VST UIs will likely become even rarer as more and more vendors move to resizable GUIs.
Sorry, I don't have any insight regarding those Overloud plugins specifically. They are pretty ugly, aren't they? I searched the registry looking for any variables that might be tweakable, but all I found was the standard stuff: display name, version numbers.