I think the display of the commit message is pracitcally hidden on the commit details page.
As it is I have the committer's avatar, then a formatted line with information about committer and date.
While this is nice the next line with the commit message's first line is completely un-formatted, and one practically has to search for it.
I think on the detail page for a commit this first line is the '<h1'> entry and should be the most prominent item on the page.
I suggest placing the commit message header above the avatar and format it the same size as 'Comments' and 'Files changed' but bold. I don't have an opinion whether the line with the link to the commits list and the committish should be above or below that heading.
The avatar should be beside the commit details (just as it is right now), and I suggest placing the body of the commit message below that and put it in a box.
Sorry to say, but this is actually better at GitHub.
See https://github.com/wbsoft/frescobaldi/commit/2a188cc21dab975491f279cee983d3088933010e as a commit of mine. When you open that page the first thing you notice is the heading of the commit message, and that is the first thing you want to see.