Bootstrap for Drupal 7 requires a lot more setting up than ordinary themes. Make sure you read and follow the instructions about installing and sub-theming Bootstrap.
If you intend to change the CSS, it is strongly recommended that you create a sub-theme with all the Bootstrap 3 source files. This method is the most dynamic and will grant you the ability to change the variables and utilize the mixins provided by Bootstrap. It entails downloading and extracting the latest Bootstrap source into your sub-theme and place it in a folder named boostrap. Note that you will need to uncomment the lines under "METHOD 1: Bootstrap Source" in your sub-theme's
.info-file if you want to use this method for sub-theming.
If you've done all this correctly, the folder structure of your sub-theme (here called
my_bootstrap) should look like this:
Bootstrap CSS is generated by means of the CSS preprocessor less, so you do not edit the CSS directly. Instead look up the file
my_bootstrap/less/overrides.less. You add your CSS overrides to this file. Then, to make your overrides effective, you can compile the file
less/style.less (which includes
overrides.less) to CSS with this CLI command (provided you're in the
my_bootstrap root directory).
lessc less/style.less > css/style.css
If you want a simpler, but less flexible method, you may use a Bootstrap CDN. It utilizes CDN Bootstrap via the base theme, but will still allow you to override the existing CSS styling in the sub-theme.
If you choose the CDN method, to override the default Bootstrap base theme settings, you copy the settings you want to change from the base theme into the sub-theme's .info-file. To override CSS, you edit the provided
css/style.css with your own overrides.
As always, after changing the CSS, remember to clear the cache to make your changes effective.