Open vivaldi://experiments -> Enable "Allow for using CSS modifications" -> Open Appearance section in settings -> Choose the folder you want to use -> Place your CSS files inside it -> Restart Vivaldi to see them in effect

gg.css

/* Auto hide bookmark bar */

#browser:not(.color-behind-tabs-off) .toolbar-addressbar {
  background: inherit;
}

#browser.tabs-top.address-top.bookmark-bar-top .toolbar-addressbar,
#browser.tabs-top.address-bottom.bookmark-bar-bottom .toolbar-addressbar,
#browser.tabs-top.address-bottom.bookmark-bar-top .toolbar-addressbar,
#browser.tabs-bottom.address-top.bookmark-bar-top .toolbar-addressbar,
#browser.tabs-bottom.address-bottom.bookmark-bar-bottom .toolbar-addressbar {
  z-index: 2;
}

#browser.tabs-top.address-top.bookmark-bar-top .bookmark-bar,
#browser.tabs-top.address-bottom.bookmark-bar-bottom .bookmark-bar,
#browser.tabs-top.address-bottom.bookmark-bar-top .bookmark-bar,
#browser.tabs-bottom.address-top.bookmark-bar-top .bookmark-bar,
#browser.tabs-bottom.address-bottom.bookmark-bar-bottom .bookmark-bar {
  position: absolute !important;
  width: 100%;
  transition: transform 0.2s !important;
  z-index: 1;
}

/* tabs-top address-top bookmark-bar-top */
#browser.tabs-top.address-top.bookmark-bar-top .bookmark-bar {
  top: 34px;
  transform: translateY(-100%);
}

#browser.tabs-top.address-top.bookmark-bar-top .bookmark-bar:hover,
#browser.tabs-top.address-top.bookmark-bar-top
  .toolbar-addressbar:hover
  ~ .bookmark-bar,
#browser.tabs-top.address-top.bookmark-bar-top
  #header:hover
  ~ #main
  .bookmark-bar {
  transform: translateY(0);
}

/* tabs-top address-bottom bookmark-bar-bottom */
#browser.tabs-top.address-bottom.bookmark-bar-bottom .bookmark-bar {
  bottom: 34px;
  transform: translateY(100%);
}

#browser.tabs-top.address-bottom.bookmark-bar-bottom .bookmark-bar:hover,
#browser.tabs-top.address-bottom.bookmark-bar-bottom
  .toolbar-addressbar:hover
  ~ .bookmark-bar {
  transform: translateY(0);
}

/* tabs-top address-bottom bookmark-bar-top */
#browser.tabs-top.address-bottom.bookmark-bar-top .bookmark-bar {
  top: 0;
  transform: translateY(-100%);
}

#browser.tabs-top.address-bottom.bookmark-bar-top .bookmark-bar:hover,
#browser.tabs-top.address-bottom.bookmark-bar-top
  #header:hover
  ~ #main
  .bookmark-bar {
  transform: translateY(0);
}

/* tabs-bottom address-top bookmark-bar-top */
#browser.tabs-bottom.address-top.bookmark-bar-top .bookmark-bar {
  top: 34px;
  transform: translateY(-100%);
}

#browser.tabs-bottom.address-top.bookmark-bar-top .bookmark-bar:hover,
#browser.tabs-bottom.address-top.bookmark-bar-top
  .toolbar-addressbar:hover
  ~ .bookmark-bar {
  transform: translateY(0);
}

/* tabs-bottom address-bottom bookmark-bar-bottom */
#browser.tabs-bottom.address-bottom.bookmark-bar-bottom .bookmark-bar {
  bottom: 64px;
  transform: translateY(100%);
}

#browser.tabs-bottom.address-bottom.bookmark-bar-bottom.stacks-on:not(.tabs-at-edge)
  .bookmark-bar {
  bottom: calc(64px + var(--padding));
  transform: translateY(100%);
}

#browser.tabs-bottom.address-bottom.bookmark-bar-bottom .bookmark-bar:hover,
#browser.tabs-bottom.address-bottom.bookmark-bar-bottom
  .toolbar-addressbar:hover
  ~ .bookmark-bar {
  transform: translateY(0);
}

.webpanel-header {
  z-index: 0;
}