ReadCube Papers supports the Citation Style Language (CSL) project who maintains a crowdsourced repository with over 9000 free CSL citation styles. If you see an issue or need an adjustment to the style, you can go to "Options" > "Manage Custom Bibliography Styles". This will take you to the CSL style editor.

Search for a style and choose edit to make the needed changes:

To save your updated version - go to "Style" and "Save Style" in the upper left menu.

You can upload the custom style via the options menu. Just make sure you give it a different name than the default CSL file (ie. MLA-your_initials.csl). Proceed back to the initial Custom Style screen in SmartCite and upload your .csl file.