Manually Editing WordPress themes

I have just started using WordPress for building websites, mainly because I want my Clients to be able to edit the websites themselves after I have built it…

Now I don’t want to just install a theme and use it, I want to customize the theme and put in my own plugins etc, maybe even rewrite some of the pages with pure HTML/CSS.

Now if I do this, will my clients still be able to use the user-friendly page editor to update content, or need I stick to the theme to enable that (without editing the theme)?

Thank you