To be able to style the Combobox, the first thing to do is to remove all the style from it, then wrap it in a div, and style this div.
You need to style DIV instead of select, since select itself does not support many CSS features per se.
/* Removendo stilos do Select */
.select-com-estilo select {
background: transparent;
-webkit-appearance: none;
-moz-appearance: none;
border: 0px none transparent;
box-sizing: border-box;
}
/* diga olá para o IE */
.select-com-estilo select::-ms-expand {
display: none;
}
/* Setando o tamanho do Select - fica ao seu criterio */
.select-com-estilo select {
width: 100%;
padding-right: 32px; /* largura da imagem (24px), acredito da "margem a direita" (olhar background-position no estilo abaixo)(4px) e da "margem a esquerda"(4px) */
font-size: 16px;
height: 32px;
}
/* A imagem abaixo possui tamanho 24px por 24px */
.select-com-estilo {
float: left;
border: 1px solid gray;
box-sizing: border-box;
background: whitesmoke;
background-image: url('data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABgAAAAYCAYAAADgdz34AAAA5klEQVRIS92V4Q3CIBCF6QQ6ihuokxgncAXdwBHcpLqBo+gGvtfkEsQD7s70T5u8tCnh++CAdkgzX8PM/LQswRrlOiGXP8t2Rv8r8iJHSkT4iGyQG3IMStj3gDyRPSUU5HDhRiQCF8YkoeCObJUReyQlXHAPClgWSlZBSQ3+Bm8naxCVNOFci/wceCVdOCtSHjSrxATXBHzXk3BQ3IrlNdWcZckbap+KlkRhJxVem4EArJIqvCfolYvtTbhF0JJ04VaBJjHBPYJcwuef3aKtvFcgEt6/tmINHhG0WGrbsn6Z7ulbOnwAN0E5GbS/JDkAAAAASUVORK5CYII=');
background-repeat: no-repeat;
background-position: calc(100% - 4px); /* "margem a direita" de 4px */
}
<div class="select-com-estilo">
<select>
<option>Primeira Opção</option>
<option>Segunda Opção</option>
</select>
</div>
To leave select as you wish, you just need to replace the image with an image like your example.
Follow the source of the image used in the Example: Flaticons - Down Arrow of Angle
I suggest reading [Ask]. Details and more objectivity are missing in your question.
– user28595
This "dashed" edge is probably an image as an edge, because the normal dashed CSS can’t be as compressed as this, which seems to be just a line looking like this from the image you’ve added to the question. But if you share the link from where you saw that border, you can confirm that.
– Chun