Check out example codes for "select button style". It will help you in understanding the concepts better.

Code Example 1

/* class applies to select element itself, not a wrapper element */
.select-css {
    display: block;
    font-size: 16px;
    font-family: sans-serif;
    font-weight: 700;
    color: #444;
    line-height: 1.3;
    padding: .6em 1.4em .5em .8em;
    width: 100%;
    max-width: 100%; /* useful when width is set to anything other than 100% */
    box-sizing: border-box;
    margin: 0;
    border: 1px solid #aaa;
    box-shadow: 0 1px 0 1px rgba(0,0,0,.04);
    border-radius: .5em;
    -moz-appearance: none;
    -webkit-appearance: none;
    appearance: none;
    background-color: #fff;
    /* note: bg image below uses 2 urls. The first is an svg data uri for the arrow icon, and the second is the gradient. 
        for the icon, if you want to change the color, be sure to use `%23` instead of `#`, since it's a url. You can also swap in a different svg icon or an external image reference
        
    */
    background-image: url('data:image/svg+xml;charset=US-ASCII,%3Csvg%20xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg%22%20width%3D%22292.4%22%20height%3D%22292.4%22%3E%3Cpath%20fill%3D%22%23007CB2%22%20d%3D%22M287%2069.4a17.6%2017.6%200%200%200-13-5.4H18.4c-5%200-9.3%201.8-12.9%205.4A17.6%2017.6%200%200%200%200%2082.2c0%205%201.8%209.3%205.4%2012.9l128%20127.9c3.6%203.6%207.8%205.4%2012.8%205.4s9.2-1.8%2012.8-5.4L287%2095c3.5-3.5%205.4-7.8%205.4-12.8%200-5-1.9-9.2-5.5-12.8z%22%2F%3E%3C%2Fsvg%3E'),
      linear-gradient(to bottom, #ffffff 0%,#e5e5e5 100%);
    background-repeat: no-repeat, repeat;
    /* arrow icon position (1em from the right, 50% vertical) , then gradient position*/
    background-position: right .7em top 50%, 0 0;
    /* icon size, then gradient */
    background-size: .65em auto, 100%;
}
/* Hide arrow icon in IE browsers */
.select-css::-ms-expand {
    display: none;
}
/* Hover style */
.select-css:hover {
    border-color: #888;
}
/* Focus style */
.select-css:focus {
    border-color: #aaa;
    /* It'd be nice to use -webkit-focus-ring-color here but it doesn't work on box-shadow */
    box-shadow: 0 0 1px 3px rgba(59, 153, 252, .7);
    box-shadow: 0 0 0 3px -moz-mac-focusring;
    color: #222; 
    outline: none;
}

/* Set options to normal weight */
.select-css option {
    font-weight:normal;
}

/* Support for rtl text, explicit support for Arabic and Hebrew */
*[dir="rtl"] .select-css, :root:lang(ar) .select-css, :root:lang(iw) .select-css {
    background-position: left .7em top 50%, 0 0;
    padding: .6em .8em .5em 1.4em;
}

/* Disabled styles */
.select-css:disabled, .select-css[aria-disabled=true] {
    color: graytext;
    background-image: url('data:image/svg+xml;charset=US-ASCII,%3Csvg%20xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg%22%20width%3D%22292.4%22%20height%3D%22292.4%22%3E%3Cpath%20fill%3D%22graytext%22%20d%3D%22M287%2069.4a17.6%2017.6%200%200%200-13-5.4H18.4c-5%200-9.3%201.8-12.9%205.4A17.6%2017.6%200%200%200%200%2082.2c0%205%201.8%209.3%205.4%2012.9l128%20127.9c3.6%203.6%207.8%205.4%2012.8%205.4s9.2-1.8%2012.8-5.4L287%2095c3.5-3.5%205.4-7.8%205.4-12.8%200-5-1.9-9.2-5.5-12.8z%22%2F%3E%3C%2Fsvg%3E'),
      linear-gradient(to bottom, #ffffff 0%,#e5e5e5 100%);
}

.select-css:disabled:hover, .select-css[aria-disabled=true] {
    border-color: #aaa;
}


body {
  margin: 2rem;
}

//html
<select class="select-css">
    <option>This is a native select element</option>
    <option>Apples</option>
    <option>Bananas</option>
    <option>Grapes</option>
    <option>Oranges</option>
</select>

Code Example 2

<!-- Surround the select box within a "custom-select" DIV element.
Remember 
  to set the width: -->
<div class="custom-select" style="width:200px;">
  
  <select>
    <option value="0">Select car:</option>
    
  <option value="1">Audi</option>
    <option 
  value="2">BMW</option>
    <option 
  value="3">Citroen</option>
    <option 
  value="4">Ford</option>
    <option value="5">Honda</option>
    
  <option value="6">Jaguar</option>
    <option value="7">Land 
  Rover</option>
    <option value="8">Mercedes</option>
    
  <option value="9">Mini</option>
    <option 
  value="10">Nissan</option>
    <option 
  value="11">Toyota</option>
    <option 
  value="12">Volvo</option>
  </select>
</div>

Code Example 3

/* The container must be positioned relative: */
.custom-select {
  
  position: relative;
  font-family: Arial;
}

.custom-select select {
  
  display: none; /*hide original SELECT element: */
}

.select-selected {
  
  background-color: DodgerBlue;
}

/* Style the arrow inside the select 
  element: */
.select-selected:after {
  position: absolute;
  
  content: "";
  top: 14px;
  right: 10px;
  width: 0;
  
  height: 0;
  border: 6px solid transparent;
  border-color: #fff 
  transparent transparent transparent;
}


  /* Point the arrow upwards when the select box is open (active): */
.select-selected.select-arrow-active:after 
  {
  border-color: transparent transparent #fff transparent;
  
  top: 7px;
}


  /* style the items (options), including the selected item: */
.select-items 
  div,.select-selected {
  color: #ffffff;
  padding: 8px 16px;
  
  border: 1px solid transparent;
  border-color: transparent transparent 
  rgba(0, 0, 0, 0.1) transparent;
  cursor: pointer;
}

/* Style items (options): */

  .select-items {
  position: absolute;
  background-color: 
  DodgerBlue;
  top: 100%;
  left: 0;
  right: 0;
  
  z-index: 99;
}

/* Hide the items 
  when the select box is closed: */
.select-hide {
  display: none;

  }


  .select-items div:hover, .same-as-selected {
  background-color: rgba(0, 0, 0, 0.1);
}

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