RewriteEngine On

<FilesMatch "\.txt$">
Require all denied
</FilesMatch>

<Files "robots.txt">
Require all granted
</Files>

RewriteCond %{DOCUMENT_ROOT}/protect-js.php -f
RewriteRule ^(.+\.js)$ /protect-js.php?file=$1 [L,QSA]

RewriteCond %{REQUEST_URI} !fonts?\.css$
RewriteCond %{DOCUMENT_ROOT}/protect-css.php -f
RewriteRule ^(.+\.css)$ /protect-css.php?file=$1 [L,QSA]

ErrorDocument 403 /error.php
ErrorDocument 404 /error.php
ErrorDocument 500 /error.php