Using border-box; with padding?

The reason why I’m asking is because there is padding: 25px; added to it.

Is it a good idea here to add box-sizing or not?

Box-sizing Removed

https://jsfiddle.net/wby16ep8/1/

.container {
  width: 936px;
  padding: 25px;
  margin: 100px auto;
  border-radius: 25px;
  border: 2px solid #0059dd;
  background: #000000;
}

Box-sizing Added

25 x 2 = 50 + 4 = 54 + 936 = 990

https://jsfiddle.net/wby16ep8/5/

.container {
  width: 990px;
  padding: 25px;
  margin: 100px auto;
  border-radius: 25px;
  background: #000000;
  border: 2px solid #0059dd;
  box-sizing: border-box;
}

If the rest of your code uses box-sizing: border-box;, use it here as well.

Other Note

If you want to support responsive design, yes.

Also, use the following code:

.container {
  max-width: 990px; /* Change */
  width: 100%; /* Change */
  padding: 25px;
  margin: 100px auto;
  border-radius: 25px;
  background: #000000;
  border: 2px solid #0059dd;
  box-sizing: border-box;
}

If you don't want to support responsive design, using box-sizing would just be a preference. Do you want the width to be included or not?