Here's an example of modifying the search input on Welcome.aspx. Please backup the Welcome.aspx and WebLinkStyles.css files before proceeding.
While probably not the best solution, you can do the following:
1. Add the attached search2.png file to the \WebLink\Web Files\images folder
2. Open Welcome.aspx in a text editor and at the bottom of the page, add the following lines of code
<script type="text/javascript">
$(document).ready(function() {document.getElementById("Search_SearchSubmitInput").placeholder='Search...'});
</script>
3. Open WebLinkStyles.css in a text editor and look for
#Search_SearchSubmitInput
{
height: 26px;
width: 254px;
}
Change that to
#Search_SearchSubmitInput
{
height: 26px;
width: 254px;
-webkit-border-radius: 15px;
-moz-border-radius: 15px;
border-radius: 15px;
background: url("images/search2.png");
background-size: 25px 25px;
background-repeat: no-repeat;
background-position: right 3px center;
}
4. Comment out or remove
.ltr #Search_SearchSubmitInput
{
border-bottom-right-radius: 0;
border-top-right-radius: 0;
margin-right: 0;
}
5. In its place, add
#Search_SearchSubmitBttn
{
display: none;
}
The end result should look like

Users will need to press the "Enter" key to search