Is there a standard icon/text that should be used?
From an accessibility perspective, there is not an agreed upon standard for an icon. The suggestions in the other answers are all good ones. Just remember that a low-vision user might not see the icon you're using so make sure it has alternate text so a screen reader will announce it properly.
Does the normal view style have sufficient contrast according to WCAG 1.4.3 Contrast (Minimum)? I'm hoping the answer is "yes" and that your high contrast view is trying to meet the AAA guideline of 1.4.6 Contrast (Enhanced).
If your normal view has low contrast and you're trying to satisfy WCAG 1.4.3 by having a style with increased contrast, just make sure that the mechanism you use to switch to high contrast has sufficient contrast itself. (That sounds a little confusing, but if your page has low contrast and you have a button to switch to higher contrast, make sure the button itself has sufficient contrast according to WCAG 1.4.3. That is, the color of the text on the button compared to the button's background color must have at least a 4.5:1 contrast ratio.)
An example of a high(er) contrast switch is on https://www.applause.com/, in the footer. They're using a switch, but you could just as easily have links between your two themes as you suggested. (Although I wouldn't use a literal link (<a>) because that conveys the wrong affordance. A "link" is an element used to navigate to another page [or somewhere else on the current page] and should not be used to perform an "action". A button is for "action".)
See also "G174: Providing a control with a sufficient contrast ratio that allows users to switch to a presentation that uses sufficient contrast". But one caveat with that recommendation, it talks about using an "alternative version" of a page as one possible solution. That is strongly discouraged by the accessibility community. Having a switch to a different theme, like you're trying to do, is a better solution.