Hugo theme Creative Portfolio has a wonderfully clean and elegant style.
However, when viewed on a desktop screen, there appears to be a one-third blank space on the right-hand side.
I’d like the content to fill the entire available space.

It was confirmed that the one-third blank space was intentionally designed that way. To modify the display settings:
theme/layouts/_default/single.html.<div class="col-lg-8"> to <div class="col-lg-X">, where X represents the column size.
For a full-page display, use X = 12.
You can also experiment with different numbers for X to suit your preference.
X = 11 in the screenshot below.
If you can’t find a solution using common keyword searches, consider navigating to the theme’s GitHub repository page.
Sometimes, the answer can be found by exploring the theme’s Issues page.