To use your own form submit handler, you should not use system_settings_formsystem_settings_form in your form. Just, instead just return your form instead of using system_settings_form.
If you are just going to save your value in your submit handler, go with system_settings_formsystem_settings_form. To
To get the valuevalues, use variable_get('your form field name')variable_get('your form field name')
. In your case,use variable_get('media_skins_test'); you can use variable_get('media_skins_test');