How to add a custom CSS Link for SharePoint 2010 and replace corev4.css. In this blog, I am going to replace COREV4.CSS to my CUSTOM_COREV4.CSS.
Step 1: I duplicated corev4.css and renamed the file as corev4_custom.css
Step 2: I made some changes to corev4_custom.css so that I can see the changes. In Ribbon Row I changed the color to RED: f7420b;
body #s4-ribbonrow{
min-height:43px;
/* [ReplaceColor(themeColor:"Dark2",themeShade:"0.9")] */
background-color:#f7420b;overflow-y:hidden;
}
Step 3: In my master page, I added this line after
<SharePoint:CssLink runat="server" Version="4"/> <SharePoint:CssRegistration name="corev4_custom.css" After="corev4.css"
runat="server"/>
This will override your default CSS file to your CUSTOM CSS. In my corev4_custom.css, I changed Ribbon Row color to RED and I can see that now.
This is how you can override your default corev4.css to your custom corev4.css file.
Aryan Nava 
In the screenshot on Step 1:
How do you navigate to that tree view of the files on your Sharepoint site? Specifically, I need to be able to access the _styles folder. It looks like you’re in Sharepoint Designer, but that’s about all I can figure out.
Thanks for any help.
Open your site in SharePoint Designer and Click on All Files Folder and you will see _styles folder.