Stuck, kindly offer me help

Can anyone tell me which property i can use to move this blue box to the center? here is the link to the project

the blue box already looks centered to me.

could you further explain your issue?

I want the blue box to decrease in size so that the background picture can also be seen

you can set the width of your blue box in the CSS

  color: white;
  font-family: halvetica;
   padding: 60px 0px 60px 30px;
  border-radius: 25px;
  box-shadow: 3px 5px 23px 0px rgba(0,0,0,0.75)
    margin: 0px 10px 0px 10px;
    width: 50%;
  margin: 0 auto;

I added the width to make it 50%

And the margin: 0 auto;
Which sets the top and bottom margin to 0 and puts an equal margin on the sides.

Give margin: 0 auto; a google to find out how it centers elements