I wasn't able to achieve that using only css. Not saying there isn't a wat. I just don't know it.

I was able to achieve your desired result, but that required DOM structure change. In other words, you would need to create a custom sub-class (js side) for your forms, and all other controls that you wish to apply this, that's a lot of work.

Basically, you would need to inject a <mark> element surrounding the label text..


This is the entire tree elements for a single cWebForm object:
Code:
<div class="WebCon_Inner  WebCon_LeftLabel">
  <label for="_df_3" class="WebCon_Float" style="text-align: right;">Order Number:</label>
  <div style="user-select: text;">
     <div class="WebFrm_Wrapper  WebFrm_HasPrompt">
        <span class="WebFrm_Prompt"></span>
        <div class="WebFrm_PromptSpacer">
           <input type="text" name="oOrderHeaderOrder_Number" value="" id="_df_3" inputmode="numeric" placeholder="" autocomplete="off" title="Order Number - New orders are assigned numbers automatically" maxlength="1048576" class="dfData_BCD" style="user-select: text;">
        </div>
     </div>
  </div>
</div>

You would have to change the html dom for the label like this:
Code:
<div class="WebCon_Inner  WebCon_LeftLabel">
  <label for="_df_3" class="WebCon_Float" style="text-align: right;"><mark style="background-color: #FFFF00">Order Number:</mark></label>
  <div style="user-select: text;">
     <div class="WebFrm_Wrapper  WebFrm_HasPrompt">
        <span class="WebFrm_Prompt"></span>
        <div class="WebFrm_PromptSpacer">
           <input type="text" name="oOrderHeaderOrder_Number" value="" id="_df_3" inputmode="numeric" placeholder="" autocomplete="off" title="Order Number - New orders are assigned numbers automatically" maxlength="1048576" class="dfData_BCD" style="user-select: text;">
        </div>
     </div>
  </div>
</div>

Result:
Click image for larger version. 

Name:	Bob_CustomLabel_Background_Webform_2.png 
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Size:	14.8 KB 
ID:	15828


Maybe others may have a better/easier solution for you..

Good luck !