The topic pages in ePUB and Kindle/Mobi eBooks use a special HTML page template that is pre-generated by Help+Manual. This is necessary because these eBook formats have special requirements and strict specifications that need to be observed to produce valid books.
You can edit the source code of the combined ePUB/Kindle template in Configuration > Publishing Options > eBooks > ePUB + Kindle: Page Template. However, please only do this if you are familiar with the eBooks specification. It is quite easy to create output that looks OK when you view it locally but creates problems later. It may be rejected by the eBook distribution platform because it doesn't "validate", or it may display incorrectly on some devices.