As you go about designing and coding your application's screens, here are a few resources that you may find useful. In rough order of importance/handiness (in my experience).
- Butterick's Typography In Ten Minutes - A ten minute read that is guaranteed to make your content look better.
- Google Web Fonts - You'll want this after reading Butterick's.
- Beautiful Web Type and Typographic Project - A curated showcase of Google Web Fonts.
- Stock Up - A search engine for free stock photos.
- 7 Rules for Creating Gorgeous UI - and part 2
- HTML5 Element List - An excellent, categorized list of HTML elements.
- Official Bootstrap Documentation - Your right hand. Bookmark this!
- Bootswatch - Cheating. Great for quick styles while prototyping.
- Bootstrap CDN - Instantaneous
<link>ing to Bootstrap or Bootswatches. - Font Awesome - Free, CSS-customizable, perfectly scalable icons for almost everything.
- Shoelace.io - A tool to help you create your Bootstrap grid layout.
- Firefox Tilt3D Extension - A tool to help you visualize what's going wrong with your layout.
- CSS3 Generator - Generate CSS box shadows, text shadows, and gradients.
- Subtle Patterns - A library of free, tileable, subtle background images.
- Adobe Color - Generate complementary colors.