FYI: The browser's Web Developer Console is super helpful to poke at things. You right-click on an element and choose "Inspect". And a tab will open with something called Inspector or Elements. And you can then mess with the styles and change them around to see what you like. It's temporary changes, so a refresh will revert everything. Somewhere below the HTML there will also be an overview with how to access the element. Unfortunately it's not super straightforward to make sense of it without prior knowledge of CSS.

I'd say the navbar is something like this:

div.navbar { background-color: aqua; }  
.nav-link { color: red; }  

I see you already got the background color of the navbar sorted. As for the text color, if you really want to change the text color for pretty much everything that isn't a link (including stuff like icons), then you could do something like this (using red as an example):

* {  
    color: red;  
}  

However, you might want to be more selective and only change the text color in certain areas.

• Post body and comments:

body {  
    color: red;  
}  

• Text in the cards on the sidebar:

.card-body {  
    color: red;  
}  

• The color of hyperlinks:

a {  
    color: red;  
}