It's a interesting discussion. I think also theme is not necessary.
Theme is same with layer plugin. The only difference is that style sheet (qa-styles.css) is essential in theme. Theme without style sheet becomes error. However, this is just one rule of the core. In other words, we can substitute theme with layer plugin.
View (HTML markup) of the MVC model is processed by qa-include/qa-theme-base.php. Blank theme below outputs no-designed page (pure HTML). This is a valid HTML page.
- Create qa-theme/Blank folder
- Create empty qa-style.css under qa-theme/Blank folder
And, when we want to change fundamental design, we need to change HTML markup. We can do that with current theme system, but overriding many functions in extended classe will degrade performance. This is one limitation of the Q2A theme system.