The best way is to configure the Theme Options properly, for example, the Typography there you can configure the Root Font Size, the body font, body color, body content font-size, and same goes for the headlines, site links, and buttons, etc.
And utilize the Font Manager and Color Manager too, that will help when changing the font-family and colors throughout the site.
Now I understand, that there are styling that is not covered in Theme Options like most of the V2 elements. I that case, this is what I advise you to do.
Style one element on your page like a button, or if you do already have a styled button, please save that as a Preset element template.
Then navigate to Template Manager > Element Defaults
There you can set the preset that you just save and set that as the default styling, do that to all elements that you commonly use.
Now if you really need to use a Custom CSS than preset, please follow Prasant's advise. Just look for the common CLASS of the element, its the class that is next to the id-classes (
e8-27), it is prepended with
But those classes will not work if use alone though, you need to make sure your CSS selector is very specific.
CSS Specificity: Things You Should Know
Hope that shed some lights,