Hello Fredika,
You will found all elements in this directory sources/template/Default/css/english/general
Like you can see, there is some css files, every css is a relation with a module. In function your module, you can change like yo want.
In your case, you must look
general/bootstrap_customize.css
general/stylesheet.css
for the header and the footer, the element is located :
modules_header
modules footer direcotries
modules_boxes (for the boxes at your left and right) : bonus !)
If I remember well, it's not advised to change inside Default directory.
In this case, create another directory, copy all the files of ExNewTemplate and css for Default directory.
In administration, you can change your default emplate directory in Design/configuration design / configuration and select your new template.
I think there is a module can do that on the fly to see your change but I don't remember the name.
Also there is an Apps can you help you to change the css via your administration, but it's not free.
The first time for me, it was not simple to understand, but after pratice it's very easy.
I hope it's not too difficult for you !
Let me know if you need more information.