I am using a modded version of an unsupported Theme (zp-skeleton)
and find following errors in my servers php_error.log

PHP Notice: Undefined variable: zpskel_ismobile in /xxx/html/zp-core/template-functions.php(4451) : eval()'d code on line 2

I am using Zenphoto 1.4.10

I would like to keep using this theme, as everything works fine.

How to change my modded theme to get rid of the errors?
How to define a undefined variable?

Well, you need to go through the code and find out why it is not defined…

Easier might be to try the theme version from our unsupported repository. Although we don't exaxtly maintain these themes in terms of functional updates we try to make them generally work. Maybe we have fixed this as I do have a far memory of some bugfixes like this.

If this works you will still have the task to find what is different with your version or alternatively to re-apply the mods to it.