Found In: Confluence Administration -> Look And Feel -> Layouts -> Main/Space/Page/Blog etc... Layout
These Layouts inherit from the Global Level, hence before customizing, the "Default" equals the changes made at Global Level.
So I recommend that these are not changed, thereby breaking inheritance; do only customazation at the Global level, or do needed customizations in Stylesheet og Macro(s)
This is where You define the look of the Space/Page/BlogPost/Comment etc.
Notice, the editing is directly and there is currently no versioning, so be carefull 
As this is not via the RTE, scipting (like in Macro(s) ) are used, and rendering a Macro is like:
$helper.renderConfluenceMacro("{jiraissues|max=100|page=10}")
Hence Macro and Wiki Markup in a mix..