"code":function(cmp, fn) { return fn.add("slds-button slds-button--brand slds-path__mark-complete slds-no-flex slds-m-horizontal--small ",cmp.get("v.backCss")); },

<button class="{!'slds-button slds-button--brand slds-path__mark-complete slds-no-flex slds-m-horizontal--small ' + v.backCss}" onclick="{!c.handlePreviousClick}">

<button class="{!concat('slds-button slds-button--brand slds-path__mark-complete slds-no-flex slds-m-horizontal--small ', v.backCss)}" onclick="{!c.handlePreviousClick}">