试了网上的方法，都不好使

#body, body {
    /*background: #f0f0f0;*/
}

html, body{
    height:100%;
    width:100%;
    /*padding:0px;*/
    /*border:0px;*/
    /*margin:0px;*/
    /*box-sizing: border-box;*/
}

img{
    border:0px
}

.bg42beff {
    background: #42beff;
}

.index-root {
    height: 100%;
    box-sizing: border-box;
    margin: 0;
    border: 0;
    padding: 0;
    background: url("../images/bg.jpeg") repeat-y center;
    vertical-align: middle;
    /*background-size: auto 100%;*/
    font-size: 0;
}

.index-title {
    width: 100%;
    height: 44px;
    overflow: hidden;
    background-color: #51a1ff;
    line-height: 44px;
    font-size: 18px;
    color: #fff;
    text-align: center;

}

.flex-center {
    display: flex;
    justify-content: center;
    align-items: center;
}

.title-return {
    width: 50px;
    height: 44px;
    background: url(../images/arrow_left.png) 50% no-repeat;
    background-size: 18px auto;
    left: 0;
    top: 0;
    position: absolute;
}

.logo-container {
    width: 100%;
    box-sizing: border-box;
    padding-left: 10px;
    padding-right: 10px;
    margin-top: 10px;
    display: flex;
    justify-content: center;
    align-items: center;
}

.logo-image {
    width: 230px;
    display: block;
    height: 230px;
}


.button-create {
    width: 100px;
    height: 50px;
    background: #51a1ff;
    color: #ffffff;
}

.mode-select-container {
    width: 100%;
    /*background: #99FF00;*/
    box-sizing: border-box;
    margin-top: 10px;
    overflow: hidden;
}

.mode-select-container::after {
    clear: both;
    content: '';
    display: block;
}

.buttons-room-container {
    width: 84.8%;
    height: 44px;
    border-radius: 4px;
    font-size: 18px;
    margin: 19px auto;
    line-height: 44px;
    /*border-radius: 20px;*/
    box-sizing: border-box;
    /*background: #99FF00;*/
    display: flex;
    justify-content: space-between;
    padding-left: 10px;
    padding-right: 10px;
    color: #ffffff;
}

.button-create-room, .button-join-room {
    width: 150px;
    background-color: #42beff;
}

.buttons-mode-container {
    width: 84.8%;
    height: 44px;
    border-radius: 4px;
    font-size: 18px;
    margin: 19px auto;
    line-height: 44px;
    /*border-radius: 20px;*/
    box-sizing: border-box;
    background: #99FF00;
    display: flex;
    justify-content: space-between;
    padding-left: 10px;
    padding-right: 10px;
    color: #ffffff;
}

.button-tourist-mode, .button-match-mode, .button-qualifying-mode {
    width: 100px;
    background-color: #42beff;
}

.buttons-videos-container {
    width: 84.8%;
    height: 44px;
    border-radius: 4px;
    font-size: 18px;
    margin: 19px auto;
    line-height: 44px;
    /*border-radius: 20px;*/
    box-sizing: border-box;
    background: #99FF00;
    display: flex;
    justify-content: space-between;
    padding-left: 10px;
    padding-right: 10px;
    color: #ffffff;
}

.button-tutorial-videos {
    
}

<div class='index-root'>
    <div class='index-title'>
        <div class='title-return left' data-v-2609e579=""></div>
        全球首款吐槽类历史游戏 - 战斗棋
    </div>
    <div>
        <div class='logo-container'>
            <img class='logo-image' src="assets/images/logo_temporary.png" />
        </div>
        <div class='game-introduce-container'>
            <div>无论你的过去、现在或者未来多么辉煌</div>
            <div>终不过是我手中的一枚棋子</div>
        </div>
        <div class='mode-select-container'>
            <div class='buttons-room-container'>
                <div class='button-create-room flex-center'>创建房间</div>
                <div class="button-join-room flex-center">加入房间</div>
            </div>
            <div class='buttons-mode-container'>
                <div class="button-tourist-mode flex-center">排位模式</div>
                <div class="button-match-mode flex-center">匹配模式</div>
                <div class="button-qualifying-mode flex-center">游客模式</div>
            </div>
            <div class='buttons-videos-container flex-center'>
                <div class='button-tutorial-videos bg42beff'>新手教程</div>
            </div>
        </div>
    </div>
</div>

我把代码抽出来了

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1">
    <meta name="viewport" content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <meta name="description" content="刘小尖">
    <meta name="keywords" content="战斗棋, 吐槽, 明星, 交友, 游戏王, 炉石传说, 棋牌, 游戏，国产，五子棋, 九宫格, 刘小尖, 历史, 独创, 简单">
    <meta name="author" content="Abbott">
    <meta name="fragment" content="!">
    <title>战斗棋 - 首页</title>
    <meta name="viewport" content="width=device-width,initial-scale=1.0,minimum-scale=1.0,maximum-scale=1.0,user-scalable=no">
    <link rel='stylesheet' href="assets/styles/base.css" />
    <link rel='stylesheet' href="assets/styles/index.css" />

    <style>
        #body, body {
            /*background: #f0f0f0;*/
        }

        html, body{
            height:100%;
            width:100%;
            /*padding:0px;*/
            /*border:0px;*/
            /*margin:0px;*/
            /*box-sizing: border-box;*/
        }

        img{
            border:0px
        }

        .bg42beff {
            background: #42beff;
        }

        .index-root {
            height: 100%;
            box-sizing: border-box;
            margin: 0;
            border: 0;
            padding: 0;
            background: url(./assets/images/bg.jpeg) 50% no-repeat;
            vertical-align: middle;
            /*background-size: auto 100%;*/
            font-size: 0;
            background-size: cover;
        }
    </style>
</head>
<body>
<div class='index-root'>

</div>
</body>

1.先看看你的index-root的高度（加个border看看是不是包着那个白边）
2.加上这个试试：background-size: 100% 100%
3.检查背景图片是不是本来就有一个白边

用你的代码，加了body{margin: 0}以后就没有问题了

建议你抽取出有关代码放进一个单独的html里。检验效果。

background-size: cover