Did your app fail in the market? It's your CSS framework. Write your CSS by hand and win over the hearts and respect of your users.
Reminds me of that time my app failed on the market because I left `// Generated by CoffeeScript` in my JS source files and became publicly scorned by my customers for taking shortcuts. I'd write a post-mortem but I'm too busy porting my Rails app to Rack to escape framework overhead like an adult.
Now sure, I could learn CSS and do all the stuff you put in the last four sentences. But designing a good-looking style with all the components I needed - grids, buttons, navbars, modal dialogs - would take far longer than unzipping Bootstrap, dropping it in my assets folder, and actually writing the bloody code. Especially since I walked out of the office Friday after emailing my supervisor a two-and-a-half page list of stuff I wanted to get done this year but couldn't.
I like the approach of just using one of the numerous form boilerplates to make your forms pretty (most time-consuming task I think), and roll the rest from scratch. You can make an entire site responsive in not much CSS at all.