file input in bootstrap css file 中修改背景颜色和风格

use this may help you

<div class="form-group">
        <div class="fileUpload btn btn-primary">
        <span>File input</span>
        <input type="file" id="exampleInputFile" class="upload">
        </div>
        <p class="help-block">Example block-level help text here.</p>
</div>  

and in style

<style>
.fileUpload {
    position: relative;
    overflow: hidden;
    margin: 10px;
}
.fileUpload input.upload {
    position: absolute;
    top: 0;
    right: 0;
    margin: 0;
    padding: 0;
    font-size: 20px;
    cursor: pointer;
    opacity: 0;
    filter: alpha(opacity=0);
}
</style>

实例配置：

                <!-- Image -->
                <div class="form-group">
                   
                   
                    <div class="fileUpload btn btn-secondary">
                        <span>选择文件</span>
                        <label for="id_image"></label>
                        <input type="file" name="pdf" id="id_image"  accept="" required>


                    </div>
                   
                </div>
 
 

  input[type=file]::file-selector-button {
    position: absolute;
    top: 0;
    right: 0;
    margin: 0;
    padding: 0;
    font-size: 20px;
    cursor: pointer;
    opacity: 0;
    filter: alpha(opacity=0);
}

　　效果：