I want to create new theme but I need to modify some files from qa-include/... for example how to modify qa-include/qa-page.php from the theme folder? if I create new qa-page.php and upload to qa-theme/Candy/qa-page.php does it work?
Definitely not possible.
Only the qa-theme.php file is loaded for themes and you can't replace it in any way with a custom qa-page.php file.
Changing qa-page.php will result in a core hack. Doing so is up to you and at your own risk
The stylish way to perform a core hack would be to use your custom plugin override, if possible. This means that if you need to change a function in qa-page.php that is overridable then a plugin override would be the less inappropriate way to go. If you need to change something that is not inside an overridable function then plugin overrides will be useless. More info on plugin overrides here: http://www.question2answer.org/overrides.php
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July 15, 2020: Q2A 1.8.5 released!