Supposing you already have Compass installed on your machine, you can easily create your own Twitter Bootstrap theme. I'm running Compass 1.0.1 (Polaris) on Ubuntu 14.04.

There is a GitHub repository that contains a guide for how to create a Twitter Bootstrap theme with Compass. Based on that, I will write here how to create and edit such a theme.

Set correct rights for gems and install bootstrap-sass

After running the command below, you will not need root access when installing gems.

sudo chown -R $USER:$USER /var/lib/gems/

Now, install bootstrap-sass:

gem install bootstrap-sass

Create the Compass project

The following command will create a compass project that contains the needed files. Replace my-awesome-bs-theme with your theme name.

compass create my-awesome-bs-theme -r bootstrap-sass --using bootstrap

Edit your theme and compile it

Enter in your theme directory (cd my-awesome-bs-theme). There you have the following structure of folders and files:

$ tree
.
├── config.rb
├── fonts
│   └── bootstrap
│       ├── glyphicons-halflings-regular.eot
│       ├── glyphicons-halflings-regular.svg
│       ├── glyphicons-halflings-regular.ttf
│       └── glyphicons-halflings-regular.woff
├── javascripts
│   ├── bootstrap
│   │   ├── affix.js
│   │   ├── alert.js
│   │   ├── button.js
│   │   ├── carousel.js
│   │   ├── collapse.js
│   │   ├── dropdown.js
│   │   ├── modal.js
│   │   ├── popover.js
│   │   ├── scrollspy.js
│   │   ├── tab.js
│   │   ├── tooltip.js
│   │   └── transition.js
│   ├── bootstrap.js
│   └── bootstrap-sprockets.js
├── sass
│   ├── _bootstrap-variables.scss
│   └── styles.scss
└── stylesheets
    └── styles.css

6 directories, 22 files

In this example, we will remove rounded corners. Open _bootstrap-variables.scss and search for border-radius-base. Set 0px for the three variables that set the border-radius value:

$border-radius-base: 0px;
$border-radius-large: 0px;
$border-radius-small: 0px;

Save the file and run compass watch in your project (my-awesome-bs-theme). This will compile the files (but will keep watching for file changes too), updating stylesheets/styles.css.

Replacing the original Bootstrap file with this CSS file in a project, you should see elements (e.g. buttons) without rounded corners:

Happy Styling!