The Back/Forward, Refresh and Home Button Box is not skinned anymore with the newest Vivaldi Version.
OK fixed it myself. Here the new addstyle Codeblock:
addStyle([
'#panels .webpanel-stack .webpanel .webpanel-header { background-color: var(--colorBgWebpanel); color: var(--colorFgWebpanel); fill: var(--colorFgWebpanel); }',
'#panels .button-toolbar { background-image: none; background-color: var(--colorBgWebpanel); color: var(--colorFgWebpanel); fill: var(--colorFgWebpanel); }',
].join(''))
})();