We highly recommend improving the default colors of your platform so that they meet contrast requirements.
Technically, having a high contrast mode is sufficient to meet WCAG conformance, but it can still create accessibility barriers for some users. If your default colors meet contrast requirements, then users with certain visual disabilities, such as color blindness, do not need to take extra steps to find a version they can perceive. The free WCAG Color Contrast Checker from Accessible Web is an easy tool to check your color combinations.
However, if you do choose to use the "high contrast mode" as the conforming alternative, you must ensure that the button that activates the mode must be fully WCAG conforming. That means that the roles, state and properties of the button element must be relayed to all users — including users of assistive technology — and the button must be operable by keyboard. Though not required for WCAG conformance, we also highly recommend making sure this button is easily located from anywhere within the app, such as by including it on all screens of your app or in a settings menu.