You are asking a question that is related to CSS practice. For such a question like this, I suggest you the way to change the look of those CSS elements.
You'd better use your Desktop version of Google Chrome or Mozilla Firefox (I prefer the latter) to browse your site.
Now right click on the area that you would like to make a change, and choose Inspect elements. You will know the element name(s) in the CSS file.
For this specific case, I think you would like to change.
.navbar {
background-color: #fff;
margin-bottom: 0;
z-index: 1010;
-webkit-transition: top 0.6s, opacity 0.6s, background 0.3s;
transition: top 0.6s, opacity 0.6s, backgroun
That is .navbar element in line 245 of the css file called donut.css.
You can even preview your changes by changing in Inspect element feature of Mozilla Firefox browser. In this case, I changed #fff (which means white) to red (because I can't remember its hex number).
If you are happy with this, you can change your donut.css file later. If you change your donut.css file, you need to clear browser cached files to see its changes.
