The content is set to 75% width, right sidebar is 25%, these 2 number is based on the container width, but the margin you set in your CSS is based on the screen width. That’s why the spaces are not even.
If you want the same spacing on the left side of the content, then the number should be 25%÷75%=33.33%. If you change the sidebar width, you’ll need to calculate the number again.
So scrap the margin-left: 25%;, use the below CSS instead: