I guess, this is more XF related question.

I have few customization done in the default style (for example, customized "sidebar_online_users" template to merge staff and member online block).

Now that I have installed EDGE, what is the best way (if there is any) to keep all the customization (existing and future) in both default style and EDGE, without doing same customization in 2 places?
You could create a simple template modification depending on how extensive your changes are. We use them here on Pixel Exit for various things, basically things we know we always wanted changed no matter what style is running the template modification will cover every style without editing it each time.

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