HRs
The problem
Unfortunately, although by default most browsers center our horizontal rules, e.g. <hr />, this is not true for e-readers using the same engine as Adobe Digital Editions (see Easy Epub/Viewing for how to get ADE if you don't have it). On those e-readers, the rules will be left justified, which will look odd. Since the way to fix them is so simple, you may want to make the few edits involved.
Guiguts default CSS
Depending on the version, the default CSS produced by Guiguts has a definition for horizontal rules like this
hr { width: 33%; margin-top: 2em; margin-bottom: 2em; margin-left: auto; margin-right: auto; clear: both; }
and several classes, for example
hr.chap {width: 65%}
The fix
Instead of defining the left and right margins as "auto", we have to set them to the correct percentage. Since the full width is 100%, we need to subtract the width of the rule to leave the percentage taken by margins, and then halve that to get the amount for each margin. For example, for the default (33%) hr, we get 100-33=67, and half of 67 is 33.5, so the replacement for the above CSS would be
hr { width: 33%; margin-top: 2em; margin-bottom: 2em; margin-left: 33.5%; margin-right: 33.5%; clear: both; }
and
hr.chap {width: 65%; margin-left: 17.5%; margin-right: 17.5%;}
Of course, you may prefer to have narrower horizontal rules - just choose the percentage width (W) that looks good, or matches the original, then use the (100 - W) / 2 formula to get the margins needed, for example
hr.chap {width: 20%; margin-left: 40%; margin-right: 40%;} hr.r5 {width: 5%; margin-left: 47.5%; margin-right: 47.5%;}
If you feel confident about it, you could make these changes permanently in your Guiguts header, so that they will be correct for all future HTML files you generate.
See the section below, to see how to take advantage of the above edits for the horizontal rules you use between chapters
Replacing Guiguts styles with classes
You might find that Guiguts has placed
<hr style="width:65%;" />
between each chapter.
If you search and replace
hr style="width:65%;"
with
hr class="chap"
and similarly for any other horizontal rules, then you will be using your newly redefined class, and your horizontal rules will center nicely on the e-readers.