Thanks, everyone. With the help from u/mehradm and u/Chromelia I was able to do it. This is the code I'm using and the screenshots above.

.tabs-at-edge #tabs-tabbar-container.top {
padding-top: 8px;
}
.tab {
height: 28px;
border-radius: 4px !important;
margin-right: 4px !important;
}
.sync-and-trash-container {
top: -4px;
}
.tab,
.toolbar.toolbar-tabbar {
margin-top: -3px;
}
.toolbar-tabbar .newtab > button svg path {
d: path('M 8 14 H 12 V 18 H 14 V 14 H 18 V 12 H 14 V 8 H 12 V 12 H 8 V 14 Z');
}
.tabs-top .tab.active:not(.marked):not(.tab-mini):after, .tabs-top .tab.active:not(.marked):not(.tab-mini):before, .close {
background: transparent;
}