Ok, sorry,
Can you try use like this?:
Uses ... ServerModule;
procedure TMainForm.UniFormCreate(Sender: TObject);
begin
UniServerModule.CustomCSS.Add( //<------------
// cor interna da treeview
'#' + UniTreeView1.JSName + '_id .x-tree-view{'+
' background-color: #2f353f;'+
'}'+
'#' + UniTreeView1.JSName + '_id .x-tree-view .x-grid-cell-inner-treecolumn{'+
' background-color: #2f353f;'+
' color: #767f8e;'+
' font: normal 0.8vw "Roboto", sans-serif;'+
' height: 40px; ' +
' padding-top: 10px;'+
'}'+
// Config da linha selecionada
'#' + UniTreeView1.JSName + '_id .x-tree-view .x-grid-row-selected .x-grid-cell-inner-treecolumn {'+
' background-color: #212529;'+
' color: #ffffff;'+
' font: normal 0.8vw "Roboto", sans-serif;'+
' border-Left: 3px solid #6e8cd7;'+
'}'
+
// Config ao passar o mouse em cima da linha
'#' + UniTreeView1.JSName + '_id .x-tree-view .x-grid-row-over .x-grid-cell-inner-treecolumn {'+
' background-color: #212529;'+
' color: #ffffff;'+
' font: normal 0.8vw "Roboto", sans-serif;'+
' border-Left: 3px solid #6e8cd7;'+
' margin: 2px #2f353f; '+
'}'
// // Altera imagem que expand o menu
// '#' + UniTreeView1.JSName + '_id .x-tree-view .x-tree-lines .x-grid-tree-node-expanded .x-tree-elbow-plus {'+
// ' background-image: url(images/elbow-minus.png);'+
// '}'+
//
// // Altera imagem que expand o menu
// '#' + UniTreeView1.JSName + '_id .x-tree-view .x-grid-cell-inner-treecolumn {'+
//
// '}'
);
end;