I guess if I did not get an answer until now, it's probably because I didn't ask clearly my question. Anyway, after hours of struggling, I found out how to deal with this.
The following tag <% Html.RenderPartial(string); %> is just a placeholder. So when the page executes, everything is merged (Master page, the view itself, the partial page, etc.). That implies I can target any element anywhere from my external css file by just keeping in mind the real location of that element in the final document.
for instance,
<div id="myDiv"> <% Html.RenderPartial("myPartialView");%></div>
inside mypartial view, i've got this,
<input type="submit" value = "Go"/>
I can access my input element this way:
#myDiv input { background:blue;}
Because, in reality, when the application executes, the whole document will present like this
<div id="myDiv">
<input type="submit" value = "Go"/>
</div>
Hope that I didn't write falsehood.Otherwise correct me.
Thanks for helping.