I am using gridelements (10.0.0) and bootstrap_grids (2.2.0) in a TYPO3 10 installation. I have included the recommended static templates of both extensions (w/DataProcessing) and the headers of the gridelements are rendered in the frontend. Where can I modify the markup of the header? I find no definition neither in gridelements nor in bootstrap_grids. The header definitions in my custom fluid_styled_content templates are not used.
Actually that's not just a TypoScript problem but a problem of Fluid root paths. In this case layoutRootPaths. Since both - gridelements and bootstrap_grids - don't come with their own layout HTML files but are based on fluid_styled_content, they will make use of the default layout provided by FSC.
This is located here: https://github.com/TYPO3-CMS/fluid_styled_content/blob/master/Resources/Private/Layouts/Default.html
So either you will have to provide another layout root path and change the layout file in use - or you will have to create a partial Header/All somewhere in your registered partial root paths.