I have a black background and white or yellow script on all my emails and internet browsing

[stcook]

I am not aware how this issue has developed.  My desktop screen appears normal colours and script 

[Dragon-Fur]

@stcook 

Collection of the obvious:

Since your windows menus and other color standards are still "normal", I would guess the "change" or setting is in your web browser.

Some browsers, for example the new Microsoft Edge (Chromium) browser, allow you to set a light or dark theme (Settings > Appearance).

Edge also supports themes - for example, "Dark Reader" theme provides the effect you are seeing.  The theme does have to be added deliberately, of course.

Email can sometimes be set to dark or light (separate from the browser setting).

You would normally know if you changed the theme on your browser.

Might be worth a check if you tend to "get into things" and push buttons.

But, what if?

it almost sounds like you have set a "high contrast" environment - although annoying to some, others find High Contrast necessary to be able to see the differences in text and backgrounds.  High Contrast on your display is a Windows theme setting (unless there is a browser extension with the same name - possible).  Normally, High Contrast would affect windows menus and other windows software and normally you'd be quite aware of it happening (since it happens only when you select it).