我有一个父View模型：

var monthVM = function (mData) {
this.Id = mData.Id;
this.Name = mData.Name;
if (mData.Days != null) {
        $.each(mData.Days, function (index, item) {
            var newDay = new dayVM(item, index);
            this.Days.push(newDay );
        });
    }
}

和dayVM：

var dayVM= function (dData, index) {
    this.Id=dData.Id;
    this.Mon = ko.observable(dData.Mon);
    this.Tue = ko.observable(dData.Tue);
    this.Wed = ko.observable(dData.Wed);
    this.Thu = ko.observable(dData.Thu);
    this.Fri = ko.observable(dData.Fri);
    this.Sat = ko.observable(dData.Sat);
    this.Sun = ko.observable(dData.Sun);

    this.didOnMon = function (item) {       
        if (item.Mon() == false) item.Mon(true);
        else item.Mon(false);
        //other stuff
    }
    this.didOnTue = function (item) {       
        if (item.Tue() == false) item.Tue(true);
        else item.Tue(false);
        //other stuff
    }
}

HTML绑定

<div class="row">
   <div class="col-9 btn-group">
        <button data-bind="css: Mon() == false ? 'btn btn-white btn-xs mt-1' : 'btn btn-dark btn-xs mt-1', click:changeMon">Mon</button>
        <button data-bind="css: Tue() == false ? 'btn btn-white btn-xs mt-1' : 'btn btn-dark btn-xs mt-1', click:changeTue">Tue</button>
        <button data-bind="css: Wed() == false ? 'btn btn-white btn-xs mt-1' : 'btn btn-dark btn-xs mt-1', click:changeWed">Wed</button>
        <button data-bind="css: Thu() == false ? 'btn btn-white btn-xs mt-1' : 'btn btn-dark btn-xs mt-1', click:changeThu">Thu</button>
        <button data-bind="css: Fri() == false ? 'btn btn-white btn-xs mt-1' : 'btn btn-dark btn-xs mt-1', click:changeFri">Fri</button> div
        <button data-bind="css: Sat() == false ? 'btn btn-white btn-xs mt-1' : 'btn btn-dark btn-xs mt-1', click:changeSat">Sat</button>
        <button data-bind="css: Sun() == false ? 'btn btn-white btn-xs mt-1' : 'btn btn-dark btn-xs mt-1', click:changeSun">Sun</button>
   </div>
</div>

我需要添加一个边框，如将深色按钮分组，一个挨着一个，但仍保持设计：按钮填充col-9 div，如

每次其中一个按钮变白色时，它都应该为其他深色的按钮调整轮廓。
我试过用

.btn-dark-custom {
   outline: 1px solid black;
   outline-offset: 2px;
}

<button class="btn btn-dark btn-dark-custom" onclick="changeWed()">Wed</button>

但它为每个深色按钮绘制边框，而不是作为一个组。我也尝试过在JS中添加border行，但我无法添加偏移量。