What I did to add a jquery plugin is in the assets.xml file I added :

<template id="myjq" inherit_id="website.assets_frontend" name="Myjq">
  <xpath expr="." position="inside">
    <script type="text/javascript" src="/theme_vish/static/src/js/jquery.cycle2.min.js" />
    <script type="text/javascript" src="/theme_vish/static/src/js/slideshow.js" />
     <script type="text/javascript" src="/theme_vish/static/src/js/backtotop.js" />
  </xpath>
</template>

and for the css I did:

<template id="mystyle" name="My style" inherit_id="website.assets_frontend">
    <xpath expr="link[last()]" position="after">
        <link href="/theme_vish/static/less/style.less" rel="stylesheet" type="text/less"/>
    </xpath>
</template>

Hopefully, this will help you.