I understand your frustration, as you sound like I felt when started down the path of copying/pasting little snippets of code. As such, I'll try to translate what Brogan was trying to say.
Otherwise, the only way to merge by hand is to make a copy of the template (e.g. using Notepad++),
1)Ok, step #1 is to open the template (i.e. form.css) by just clicking on the name of it. On the resulting page, go to the Template Contents box and select all text (i.e. Ctrl-A) and copy it.(i.e Ctrl-C) Now, paste that text into a new file to look at in just a little bit.
revert the template,
2)Reverting a template, puts it back into stock condition just like it was when it was brand new.(i.e. before any editing was performed on it) So, when you revert the template it will go back to just how it was issued and you will
lose any custom changes that have been made to it.
Note: Immediately, any changes that have been made to that template will be lost and your board may look a little different until those edits are re-added to the newer version of the template.
compare the saved copy and the reverted template
3)Now comes the time to put on your thinking cap. You will have to sift through the template which has been reverted back to stock condition -versus- your saved copy of your modified template now in Notepad. They should be pretty similar, but keep looking and you should begin to see some differences…which are most often some additional lines of code.
and edit it to match.
4)OK, now that you know what additional lines of code were inserted (or even possibly removed from) into your modified template you will need to copy those lines of code (i.e. Ctrl-C) out of your modified template that you have in Notepad and paste them into the newer v1.5.8 style template in XenForo.(or remove them from the newer v1.5.8 template, if your comparison indicates a line of code was removed)
This is sort of a hunt-n-peck exercise, as the v1.5.8 template may be a little different than your previous (form.css) template and you'll just have to sift through it looking closely at things.
The good news is, with the ability to revert a template back to it's stock condition, you always have a bread-crumb-trail back out of the woods to put things back into
stock condition.(i.e. As issued by XenForo)
So, maybe that helps to demystify things a bit for you…as well as gain GREAT respect for the Coders of XenForo and all it's add-ons!