Webpages can already customize the cursor themselves.

Can you describe what the motivation for this is? I presume it is for accessibility.

This feature can be either for accessibility or simply for theme and appearance customization. This is why I am unsure whether or not this should be under "Accessibility" or "Appearance". Before implementing, we’ll need to discuss where we believe this menu should be implemented and how it should look, maybe posting some example images. Perhaps we should place the menu under “Appearance” since it is technically appearance, but include a link to cursor themes or appearance under accessibility.

Per your comment, a developer had made an extension with that in mind, however, it has limitations (see https://chrome.google.com/webstore/detail/custom-cursor/iclinnjcnpihmbogglkigdkhkeajbcck?hl=en-GB )

In summary, it does not work on the UI, only in the browser window. Under "conditions" (see link):

"
Conditions:
- Any tabs open when you install the extension must be reloaded so the extension can work.
- the extension can not work on the chrome web store.
- the extension can not work on the chrome ui. This includes the bookmarks bar, the url bar, and the tab selection at the top of the screen." (end quote)

Additionally, the extension can only change the cursor and will not allow adjustments for the entire cursor theme, however, this functionality may be feasible by improving the respective app. It appears, though, that functionality on UI is outside of the technical limitations of any app or webpage, therefore, requiring implementation to the OS.

This page may give us a roadmap to implement this feature in an update. I would otherwise have to enter developer mode... https://askubuntu.com/questions/933073/how-can-i-change-the-mouse-pointer-theme-from-the-command-line

Also, if we do implement this feature, we need to discuss supported formats. We need to at least have .png support. We also need to add support for any Linux cursor file formats. A roadmap (and examples) for that includes the following resource: https://askubuntu.com/questions/375216/opening-cursor-files-in-a-graphics-editor

In terms of accessibility, we may also add preset options for cursors (and cursor themes, of course) with highlights or bright colors, for example, that the user can select without having to upload a file. However, again, the question comes as to whether or not these options belong under Appearance, Accessibility, both, if one place should link to the other, or, if shortcuts should be provided for important accessibility options.

Additionally, for convenience of the users, we can add capability of Chrome browser themes to:
- Change cursor theme in Chrome OS
- For Chrome Browser users, to change cursor theme within the browser, but to also (as discussed above) work on the bookmarks bar, address bar, etc, which seems to be outside of the technical limitations of webpages or apps.

In terms of accessibility concerns, for example, if a user were to select a "high contrast" theme (meaning a custom high contrast theme from the webstore rather than system default), it could also include an accommodated cursor theme.

Let me know what you think.