The caret color is based on your OS theme and I'm not aware of a way to override that directly. You can override the text colors used by pages, and if the cursor does not take on that new color, the visibility would be better, but that's a rather drastic solution. Maybe not so bad if you're just changing form controls, but if you are trying to change "rich text" editors such as webmail editors, then it might be hard to target it precisely.This doesn't seem to work because when the text is white on a dark background, the caret turns white.
Note: I would use the Stylish extension for experimenting with changes. You also can right-click an element and use Inspect, then turn on the style pane using the button in the lower right corner to play with the style rules.