Add a node under tabs node for ur sidekick with the name 'parentConfig' and jcr:primaryType as 'nt:unstructured' , to this node add property name 'icon' and value 'linux'. it will add an icon to the tab. Similarly, if u add a child nodes 'items' under 'tabs' node, and under 'items' node add a 'title' node, and add properties:
sling:resourceType:granite/ui/components/coral/foundation/text
text:name of the title
granite:class-sidepanel-tab-title