Is there a thorough guide for what each color setting modifies in the "customize theme" color menu?
Everything you want to change would have to be done using SkinStudio. Example to change the Clock color and font. Open the skin in SKS (skinstudio) Click on Edit Startmenu and taskbar. when the new window opens click on Edit Horizontal Taskbars. Go down and click on Clock Text. Here you can change the font and color from a drop down menu. You can also make new fonts and new colors to use if you wish. You can do that by opening the "change skin colors" and Change skin fonts.
The searchbar text can't be changes.
When you say change the taskbar text I assume you are talking about the button text as it's the only text on it. Same location as about "Edit Horizontal Taskbars" Except goto Taskbar buttons/Buttons. At the bottom click on "Text" You will see the first one is "Normal" Use the dropdown menu to change the other states.
Hope this helps you a bit.