Do not override button background image

Use a weaker CSS declaration for the background color of buttons. This enables the use of background images in buttons without the need for an !important declaration to override owncloud/core CSS.

This change is necessary to fix https://github.com/owncloud/contacts/issues/351 after the introduction of https://github.com/owncloud/core/pull/6912 and https://github.com/owncloud/core/pull/7310

CC @jancborchardt