Razor has nothing to do with app styling and therefore there is no documentation. ASP.NET templates use Bootstrap so if you are using the default template then overloading a Bootstrap style would be covered by the Bootstrap documentation as it isn't related to ASP.NET/Razor. The only "customization" that ASP.NET does is define a side-wide style CSS file where you can then add your custom styling that would be applied in addition to the Bootstrap styling.
I'm not 100% sure but in CSS, to reset a rule to the default value then use the revert value. But note that revert simply undoes any styling changes to the current element. It might still inherit values from the parent. The initial resets a rule to the default value but I believe then ignores custom user styling as well which is not desirable. It also doesn't work for all properties. Finally you have the unset value which basically acts like revert or initial depending upon the type of property.