Tom, This might be a lot easier than I thought. The new version has the capabilities for me to copy the main theme and make "child" copy of it. That allows me to keep making any changes in the parent and those will flow down to the child. I can them put specific child settings in that will override the parent. An example is changing the font colors. I created a high contrast version and just enabled it. The only changes I made so far is changing the forum names colors to black and change the thread names to black. See what you think? To change themes, click the Default Style button in the bottom left of your web browser.
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From there you should get a pop-up for the Style Chooser. From there select high contrast and you should be good to go.
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Let me know what you think and if anything you would like to see changed.