Try to give css:

1. for that just create a css file example.css in your custom module

add your css code there

2.make an asset file asset.xml and add css in backend assets

     eg:

        <template id="" name="style1" inherit_id="website.assets_backend">
             <xpath expr="." position="inside">
                 <link href="yourcssfilepath" rel="stylesheet" type="text/css"/>
             </xpath>
         </template>     

3. add asset file in manifest , then upgrade 

you can set you own code in css for the tree view and use its proper classes

eg css code:

.oe_list {    background-color: white !important;

}