Portfolio websites best work if they are simple and minimalistic, so that they won't distract visitors from your work, which is the most important aspect of your portfolio. And if you want your website to remain minimalistic but a monochromatic background at the same time doesn't satisfy you, we have a compromise.
The SubtlePatterns.com website offers a variety of simple but attractive visual patterns, that you can download for free and use them for example as a background image of your website. SubtlePatterns.com allows you to search through specific patterns using keywords and to also see each of the patterns in action using the preview button.
After you choose your pattern, you can legally get it for free by clicking on the download button, which will download a .zip file to your computer (that you can open for example with the 7-Zip programme). You can then upload the downloaded image directly to your administration under the Settings folder, either to the Website background, or the Website contents area.
However remember that if you do set the subtle image as a content background, you need to take into consideration your ptychs (if you have any on your portfolio), because ptychs use monochromatic borders between images that might not look very nice with your image patterns. On the other hand, you can completely remove these borders while creating your ptychs.