If you make CSS modifications in the style.css, they will be overwritten if you install a new version of a theme. To avoid such problems it is recommended to make modifications in an alternative .css file. The following instructions show you how you can prepare your theme to do so.
Open a (text)editor, create a new file and save it as .css (in our example local.css) in the root directory of your theme (sites/all/themes/themename).
Use a (text)editor to open the theme .info file (sites/all/themes/themename/themename.info). Add the following line after the last line ending with.css:
stylesheets[all] = local.css
Go to Configuration > Development > Performance and "Clear all caches".