Is there a standard symbol / text that should be used?
From the point of view of accessibility, there is no agreed standard for a symbol. The suggestions in the other answers are all good. Remember that a user with poor eyesight may not do this see If you use the icon, make sure that there is alternative text for a screen reader to correctly advertise.
Do it normal view Style have sufficient contrast according to WCAG 1.4.3 Contrast (minimum)? I hope the answer is "yes" and yours High contrast view tries to meet the AAA policy of 1.4.6 Contrast (Enhanced).
If your normal view has a low contrast and you try to satisfy WCAG 1.4.3 with a style with increased contrast. Just make sure that the mechanism you use to switch to high contrast has sufficient contrast itself. (This sounds a bit confusing, but if your page has low contrast and you have a button to switch to higher contrast, make sure that the button itself has sufficient contrast in accordance with WCAG 1.4.3 Color of the text on the button compared to the background color of the button must have at least a contrast ratio of 4.5: 1)
An example of a high contrast switch can be found at https://www.applause.com/ in the footer. You use a switch, but you could just as easily have links between your two topics as you suggested. (Although I would not use a literal link () because that gives the wrong permission. A "link" is an element that navigates to another page [or somewhere else on the current page] and should not be used to perform an "action". A button stands for "action".)
See also "G174: Control with sufficient contrast ratio that allows users to switch to a presentation that uses sufficient contrast." A disadvantage of this recommendation, however, is the use of an "alternative version" of a page as a possible solution. This is badly discouraged by the accessibility community. If you want to switch to another topic as you try, this is a better solution.