Below is mentioned in jQuery Mobile Docs

If no theme swatch letter is set at all, the framework uses the "a" swatch (black in the default theme) for headers and footers and the "c" swatch (light gray in the default theme) for the page content to maximize contrast between the both.

All items in containers inherit the swatch from their parent. Exceptions to this rule are the listdivider in listviews, the header of nested list pages, and the button of split button lists. Those default to "b" (blue in the default theme). Count bubbles default to "c" (silver in the default theme).

My questions are

1) Why jquery mobile doesn't use a single theme. Why something from "a" something from "b" and "c"?

2) Because of it's using multiple theme it's not possible to make a single custom theme. For example if I make "g" for my custom design. Is there a way to force it to use single swatch "a" or "b" or "c" only so i can make custom theme using any of them.