Plenty of people know how to fix errors, it can be time-consuming to track down even with full access to the code and can be like trying to diagnose car trouble without being able to open the bonnet when you only have access to the front end.
Several of the errors you are getting seem to be related to incorrectly nested xhtml and the messages shown in the validator does give some clues
As an example, the first error I can see is:
Line 62, Column 35: document type does not allow element “div” here; assuming missing “li” start-tag
<div class=”searchdivleft”>
To validate, all this stuff needs to be enclosed within in a list item so it needs a <li>
before the first <div>
and a </li>
after the last </div>
You could probably replace that div with an li. If there’s a file in your theme called searchform, it may be in there or it may be in your sidebar template.
<div class="searchdivleft">
<div class="searchdivright">
<form id="searchform" method="get" action="">
<input type="text" name="s" id="s" value="Search..." onfocus="if (this.value == 'Search...') {this.value = '';}" onblur="if (this.value == '') {this.value = 'Search...';}" />
<input type="submit" name="searchbtn" id="searchbtn" value="Go" />
</form>
</div>
</div>
It does look as if the theme you have built on had at least some of the same errors you got before you made any change. Perhaps the developer has an improved version available?
https://validator.w3.org/check?uri=http%3A%2F%2Fwp-themes.com%2Fmulti-color%2F&charset=%28detect+automatically%29&doctype=Inline&group=0