Don't think of Error Messages as unreadable ancient glyphs and read it out to yourself, like so:
Fatal error. This means that PHP can not process the error, it doesn't know how to handle it. There are some errors PHP can just skip past, but anything that triggers a Fatal error is, well, fatal (for the script (and perhaps a developer's sanity?))
Now, we need to figure out WHY it's dying. "Class BUILDR_OPTIONS not found in [script location] on [line number]".
If you go to that directory C:\xampp\htdocs\wordpress\wp-content\plugins\buildr-features\inc\ and open up that file functions-customizer.php, and go to line 24 - you'll see that it's trying to instantiate a class called BUILDR_OPTIONS, probably by using something like the following:
$buildr_options = new BUILDR_OPTIONS();
The error mentions that the BUILDR_OPTIONS class hasn't been found (which means it's not yet defined).
Since this happened when you changed Themes, I'd wager that your old theme defines that class (is it by chance named something like Buildr Theme?). So, you'll either have to reactivate that theme, or disable the buildr-features plugin. If you can't access the dashboard, you can disable it by editing the folder name. The standard "I'm renaming this plugin to temporarily disable it" practice I've seen the most is to rename it with a -x suffix, so in this case rename it to buildr-features-x. That will shut the plugin down, and renaming it back will cause it to reactivate.
So your easiest options are:
