How determine what CSS selector to use from my HTML source code? -

The wordpress theme I'm using is acting, so I edited my site using style sheets. I am trying to do (I have 0 experience with HTML or CSS) I am getting a googling process to search for selector names, I should use every element within my site to take a lot of time, so I try the selector and To begin recognizing the source code has begun.

For the example below, I want to change the background color of the signup form button.

  & lt; Div class = "notif" & gt; & Lt ;! - Start the MailChimp signup form - & gt; & Lt; Div id = "mc_embed_signup_appstage" & gt; & Lt; Form Action = "http://HUDKingPro.us7.list-manage.com/subscribe/post?u=0c2c35acf6b36619d521176a9&id=8d4d76b2db" method = "post" id = "mc-embedded-subscribe-form_appstage" name = "MC -added-subscription-form "class =" validate "target =" _ blank "& gt; & Lt; Input type = "email" value = "" name = "EMAIL" class = "email" id = "mce-EMAIL_appstage" placeholder = "Enter your email address" is required & gt; & Lt; Input type = "submit" value = "notify me!" Name = "subscribe" id = "mc-embedded-subscribe_appstage" class = "button" />   
 So far I have come to this using the selector I have identified, but this does not work, is there a method of identifying the CSS selector with the HTML source code?    #notif input {background: # 1aaaa 18; }   
 If it is superficial, then I have an app to launch for a week, so I had inadequate time to develop a complete understanding of CSS.   
 
 
 Have you tried using the button that is in the button? I.e.  .button   
 Then it will be:  
  .notif .button {background: # 1A1A18; }   
  and 
 in your div , so you are invalid for CSS with id  #  and classes for ID and  #  for . Use .  
  Edit   
 This is taking this stylesheet  
 Edit background with 76 or by colors:  
 This is the class or input that you want to edit:  
  .notif input # mc-embedded -subscriber_plastes {background: # 1a1a18! Important; }   
 Appeal for the above - Editor.