Simply go to your Website and click "edit" > "HTML-/CSS-Editor". Then coose in the top left corner "SCSS (CSS)" and the file "website" for example in the menu. At the top of the code write something like this: 

/* Global color defaults*/
$primary-color: #000000 !default;
$secondary-color: #ffffff !default;
/* *** */

Now you can use the global colors everywhere you want in the css code. For example:

.carousel-content {
color: $primary-color;

Hope it helps. Best regards. 

You can add custom sccs in odoo online version. 

Currently there is not a solution for this. Odoo 11 is bringing an option to edit CSS which will allow you this option. Online I do not believe will give this option but Enterprise self-hosted will. We ran into this same issue using online and switched to self so we could more easily access this code. The other option for online is to pay for service pack and have Odoo support do the updates for you.

Video Link: https://youtu.be/amHpruXQMOg

Module Link: https://github.com/hareshkansara/odoo_tutorial/tree/13.0/odoo_theme_backend