Error message if chosen theme does not have "meta:theme" tag.

client/homebrew/navbar/error-navitem.jsx

@@ -116,6 +116,19 @@ const ErrorNavItem = createClass({
 			</Nav.item>;
 		}
 
+		if(HBErrorCode === '10') {
+			return <Nav.item className='save error' icon='fas fa-exclamation-triangle'>
+				Oops!
+				<div className='errorContainer' onClick={clearError}>
+					Looks like the brew you have selected
+					as a theme is not tagged for use as a
+					theme. Verify that
+					brew <a className='lowercase' target='_blank' rel='noopener noreferrer' href={`/share/${response.body.brewId}`}>
+						{response.body.brewId}</a> has the <span className='lowercase'>meta:theme</span> tag!
+				</div>
+			</Nav.item>;
+		}
+
 		return <Nav.item className='save error' icon='fas fa-exclamation-triangle'>
 			Oops!
 			<div className='errorContainer'>

client/homebrew/pages/errorPage/errors/errorIndex.js

@@ -167,6 +167,14 @@ const errorIndex = (props)=>{
 		**Requested access:** ${props.brew.accessType}
 
 		**Brew ID:**  ${props.brew.brewId}`,
+		
+		// Theme Not Valid
+		'10' : dedent`
+		## The selected theme is not tagged as a theme.
+		
+		The brew selected as a theme exists, but has not been marked for use as a theme with the \`theme:meta\` tag.
+		
+		If the selected brew is your document, you may designate it as a theme by adding the \`theme:meta\` tag.`,
 
 		//account page when account is not defined
 		'50' : dedent`

server/homebrew.api.js

@@ -294,6 +294,8 @@ const api = {
 
 				currentTheme = req.brew;
 				splitTextStyleAndMetadata(currentTheme);
+				if(!currentTheme.tags.some(tag => tag === "meta:theme" || tag === "meta:Theme"))
+					throw { brewId: req.params.id, name: 'Invalid Theme Selected', message: 'Selected theme does not have the meta:theme tag', HBErrorCode: '10' };
 				themeName   ??= currentTheme.title;
 				themeAuthor ??= currentTheme.authors?.[0];