[improvement] Add a green background to the confirmation message that shows up after filling in the add app form

[aniri]

Currently we have the following two cases after the user fills in the add app form data and submits it:

• success message on the top of the form
  
• error message on top of the form
  

We should make them more similar:

• make the success message also show on one line and same font size as the error one
• make both texts bold
• add a light green background to the success confirmation message
• increase the top and bottom padding of the background surrounding the messages to make them more visible.