Customize Theme "Tides" with custom.css

Hello Forum,

I recently acquired a theme called Tides and can’t figure out how to change the background colors of certain parts of the page.
I have been told that I can do this by copying code from the style.css to a custom.css and editing it there and it will override settings in the style.css…

I inspected the elements i want to change in chrome: =

<div class=”section-marker” id=”archiv”></div>
<div class=”section-marker” id=”reel”></div>

But I am not sure what code I need to add to the custom.css file in order to change these elements (segment headers?)

Can someone please help me find the right code to change the background color of these items? Or maybe even set their opacity to 50%?
The page i’m working on is http://www.dvfx.de

Thanks in advance!

Cheers, Fxam

https://wordpress.org/plugins/css/