Sidebar position

Hello Yannick,
thank you for reporting this.

In order to change the Sidebar position, just make the following 2 changes inside your style.css (skeletontheme/css/style.css):

style.css Line 156
Change from
#wrap #content { float:right; }
To
#wrap #content { float:left; }

style.css Line 281
Change from
#wrap #sidebar { float:left; }
To
#wrap #sidebar { float:right; }

Adding style rules inside a local.css

You have always the option to support a local.css file where custom CSS rules (as the above 2) and modifications could be placed. Following this practise, there is no worry if the next theme update will override all your custom changes.
This can be done in 3 simple steps:

1. Create an empty .css file under the /sites/all/themes/skeletontheme/css/ path and name it as you wish. local.css is always a good choice.
2. Edit the skeletontheme.info file and add a line like stylesheets[all][] = css/local.css after stylesheets[all][] = color/colors.css
3. Clear all cached data.
   
   This step is important in order to force your template to be informed for the changes you have made in the template files. To do this go to Administer » Site configuration » Performance » Clear cached data

Thanks
George