I am trying to tweak blocks with custom CSS. I wonder where should I declare and place CSS classes?

Accepted answer

I think that the best practice in Backdrop development would be to declare and define your CSS classes in the theme. Ideally, you would create a sub-theme for any custom css. 

If you have some small tweaks to the CSS, you can use the CSS injector module. I have rarely used this module so I can't say much about how to use it. But, if you don't want to create custom css files in the theme or if you are not able to (if you are a site editor without access to the theme), you ought to try this module. 

Comments

I think that the best practice in Backdrop development would be to declare and define your CSS classes in the theme. Ideally, you would create a sub-theme for any custom css. 

If you have some small tweaks to the CSS, you can use the CSS injector module. I have rarely used this module so I can't say much about how to use it. But, if you don't want to create custom css files in the theme or if you are not able to (if you are a site editor without access to the theme), you ought to try this module. 

Thank you for in depth response! The CSS injector works excellent and it is exactly what I was looking for. Creating a theme/sub-theme would be a more solid approach but I am afraid I don't have much time to spend on it. Anyway it is good to know about it too.