Hi Mathieu,
You're not obliged to use ClientBundle, this is only an optimization to remove unused styles and shorten the css names.
I suggest you do a right click on your page (show page source) to check why your css is not loaded. (There is no complicated procedure to include a css, you must have an easy to find error).
Also it's better for "beginner questions" to ask them on StackOverflow, people tend to be more present there and answer you faster. Also notice that if your question is too easy to solve, people tend not to answer, so try to fight a little with your problem, google it, and only if still not found ask. And don't stop searching for an answer when you ask, you might loose time because no one answers
Best regards,
Zied