3. Configure CSS Injector
Label text field = enter a name for your CSS Injector.
Code text area = add your CSS classes here. Add as many classes as you need. Make sure you annotate things well.
Media select field = all is default, leave this as is.
Preprocess CSS checkbox = checked by default. This aggregates CSS into a single file which results in a performance boost. Keep it checked.
There are many different conditions that are available. Additional conditions may also be added as more contributed modules are enabled.
For now, we'll just set the theme condition as it's most relevant.
Click the Current Theme vertical tab.
Theme select list = Bootstrap.
Click the Save button.
Note: if you would like to continue editing your CSS after saving, click the Save and Continue Editing button.
Flush the cache.