Make an element take all available width

HTML-CSS
1

Hey guys, i making a project for practice and i cant make the search input take all available with. Can someone pls help me?
sorry my code is kind of big. its in the end of css form{..} and #searchInput

https://codepen.io/oshikurou/pen/xNOObN?editors=0100

2

Remove auto (line 146):

#second-nav{
  ...
  width: 100%; /* no auto */

Add width to form (line 172):

form {
  ...
  width: 100%;
  ...
}