color titles pages

your headlines do not use consistent classes as far as I see it so this will end up in a lot of work.
I would suggest to wrap every headline in a class.
i.e. sidebar-title, post-title etc.
this way you can then simply add

.sidebar-title, .post-title {
 color: red;
padding:10px 0 ;
}

I have added it in my stylesheet but nothing happens.

I wonder is there a code to change a particular color into another for the WHOLE website?
Thanks for your reply.

you first need to go through your theme files and add that class to each <hX> tag. It can then be modified with custom css settings. Only adding it to style.css won’t change it.
here are some video tutorials:
https://courses.tutsplus.com/courses/30-days-to-learn-html-css