[GeoffR]

I don't think there is a universal stylesheet that will work very well, there are just too many different ways of creating epubs for one stylesheet to work on them all.

These are the common things that publishers do that prevent things being adjustable:

Font size: Specifying font sizes in absolute units px, pt, in, etc. causes problems, not just on Kobo devices. To fix, convert them to relative units em, %, etc. or small, medium, large, x-large, etc.

Line spacing and justification: Specifying line-height or text-align properties at paragraph level will prevent them being adjusted at paragraph level. To fix, delete all line-height and text-align:left / text-align:justify properties at paragraph level. For best results, leave the line-height property intact for headings, table of contents entries, etc. and leave text-align:left intact for headings, poetry and other special types of paragraph, specify a default line-height and text-align at body level, and let the ordinary paragraph level elements inherit from body. This will give an epub that works properly on all devices, but also allows Kobo devices to adjust the line spacing and text alignment.

Another thing that prevents line height being adjusted is use of the font: property. To fix, replace font: with seperate font-family and font-size properties.

e.g. this form will prevent line spacing, alignment, and font size from being adjustable:

h2 {line-height: 1.5; font-size 18pt; text-align: left;}
p {line-height: 1.5; font-size: 12pt; text-align: justify;}

replace it with something like this and the main text will be fully adjustable and the headings will scale properly, while the line spacing and alignment of the headings will remain as they were:

body {line-height: 1.5; text-align: justify;}
h2 {line-height: 1.5; font-size: 1.5em; text-align: left;}
p {font-size: 1em;}

Obviously real stylesheets are more complicated, but it tends to be the same problems just repeated in different ways, so once you have fixed a few it becomes easy to fix the rest.

Another possibility to consider is converting the epubs to kepubs for sideloading. The kepub reader is more heavy-handed in how it enforces the adjustment settings, so line spacing and justification settings will often be overrided for kepubs even when they wouldn't be for epubs. (But the downside to this is that sometimes they are too heavy-handed, and some things which shouldn't be overridden are.)

Edit: There is also a patch (see the developer's subforum) that will allow the line spacing and justification settings to work for some epubs where they wouldn't before, it addresses the most common reason that these adjustments don't work, but it doesn't fix all the possible problems that epubs can be created with.