This is not the most attractive website.
So redesign it to look nicer!
...is to use Bootstrap or Foundation (or both!) without touching anything but the HTML.
That's right: no custom stylesheets. The exception is using SASS to, say, make .form-control apply to all input. (So to be really specific: no customizing with CSS properties.)
But this is a "soft challenge". The important thing is for you to flex your design muscles. This homework will be considered "complete" as long as it's clear you made some effort to redesign the site using Bootstrap and/or Foundation.
...you think it's done.
Unfortunately, beauty is subjective, so there will never be a clear stopping point when it comes to design.
The best way to check your progress is to ask someone else to look at your site for 10 seconds and tell you what they don't like. People have a lot of trouble defining what looks good or bad, but everyone knows it when they see it.
Backbone relies on some of the classes and IDs that are in the index.html file. For instance, if you were to delete id="grumbleTemplate", Backbone wouldn't know where to find the template for Grumbles.
Deleting any of the IDs or classes that are present when you first clone down this repo will affect the app's functionality. However, feel free to add classes. For instance, it's no problem to turn <button class="new">Create Grumble</button> into <button class="new btn btn-xs btn-danger">Create Grumble</button>.
...but it's not "right". There's no "right" -- that was just my take on Bootstrapping Grumblr, and I am by no means a visual designer. Feel free to refer to it to see how different classes are used, but please do not just copy it.
...do your usual forking, cloning, changing, pushing, and pull-requesting.
...is if you also push your code to a gh-pages branch on your fork! Then you and anyone you decide to share your code with will be able to see your Grumblr without having to clone it just by going to:
https://[your_username].github.io/grumblr_css_redesign