下拉列表（Select）
使用重置的CSS，重定义原生的下拉列表外观。
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默认
自定义code><select>菜单只需要一个自定义类.form-select就可以触发自定义样式。自定义样式仅限于select的初始外观，并且由于浏览器的限制，无法修改<option>。
<select class="form-select" aria-label="Default select example">
<option selected>Open this select menu</option>
<option value="1">One</option>
<option value="2">Two</option>
<option value="3">Three</option>
</select>大小
Y您也可以选择从小型和大型定制选择，以匹配我们同样大小的文本输入。
<select class="form-select form-select-lg mb-3" aria-label=".form-select-lg example">
<option selected>Open this select menu</option>
<option value="1">One</option>
<option value="2">Two</option>
<option value="3">Three</option>
</select>
<select class="form-select form-select-sm" aria-label=".form-select-sm example">
<option selected>Open this select menu</option>
<option value="1">One</option>
<option value="2">Two</option>
<option value="3">Three</option>
</select>还支持多文件属性multiple ,按住Ctrl键，可以一次选择多个选项
<select class="form-select" multiple aria-label="multiple select example">
<option selected>Open this select menu</option>
<option value="1">One</option>
<option value="2">Two</option>
<option value="3">Three</option>
</select>支持大小属性 size :
<select class="form-select" size="3" aria-label="size 3 select example">
<option selected>Open this select menu</option>
<option value="1">One</option>
<option value="2">Two</option>
<option value="3">Three</option>
</select>禁用
在select上添加 disabled属性，使其显示为灰色，并删除指针事件。
<select class="form-select" aria-label="Disabled select example" disabled>
<option selected>Open this select menu</option>
<option value="1">One</option>
<option value="2">Two</option>
<option value="3">Three</option>
</select>Sass
Variables
$form-select-padding-y: $input-padding-y;
$form-select-padding-x: $input-padding-x;
$form-select-font-family: $input-font-family;
$form-select-font-size: $input-font-size;
$form-select-indicator-padding: $form-select-padding-x * 3; // Extra padding for background-image
$form-select-font-weight: $input-font-weight;
$form-select-line-height: $input-line-height;
$form-select-color: $input-color;
$form-select-bg: $input-bg;
$form-select-disabled-color: null;
$form-select-disabled-bg: $gray-200;
$form-select-disabled-border-color: $input-disabled-border-color;
$form-select-bg-position: right $form-select-padding-x center;
$form-select-bg-size: 16px 12px; // In pixels because image dimensions
$form-select-indicator-color: $gray-800;
$form-select-indicator: url("data:image/svg+xml,<svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 16 16'><path fill='none' stroke='#{$form-select-indicator-color}' stroke-linecap='round' stroke-linejoin='round' stroke-width='2' d='M2 5l6 6 6-6'/></svg>");
$form-select-feedback-icon-padding-end: $form-select-padding-x * 2.5 + $form-select-indicator-padding;
$form-select-feedback-icon-position: center right $form-select-indicator-padding;
$form-select-feedback-icon-size: $input-height-inner-half $input-height-inner-half;
$form-select-border-width: $input-border-width;
$form-select-border-color: $input-border-color;
$form-select-border-radius: $border-radius;
$form-select-box-shadow: $box-shadow-inset;
$form-select-focus-border-color: $input-focus-border-color;
$form-select-focus-width: $input-focus-width;
$form-select-focus-box-shadow: 0 0 0 $form-select-focus-width $input-btn-focus-color;
$form-select-padding-y-sm: $input-padding-y-sm;
$form-select-padding-x-sm: $input-padding-x-sm;
$form-select-font-size-sm: $input-font-size-sm;
$form-select-padding-y-lg: $input-padding-y-lg;
$form-select-padding-x-lg: $input-padding-x-lg;
$form-select-font-size-lg: $input-font-size-lg;