I already tried to install the theme "Isola" but that don't help. Searching for "isola-wpcom" on google also gave me no results.
I think you need this:
Really simple some themes do have some requirements, either other themes or plugins. However, after some checking, I cannot find the theme "celsius".
If we check the source code from https://celsiusdemo.wordpress.com/
And precisely the style.css you will get the feedback the theme name is Isola.
Theme Name: Isola
Theme URI: https://wordpress.com/themes/isola/
Author URI: https://wordpress.com/themes/
Description: Isola is a fresh, clean slate for your best works, perfect for showcasing your writing, photographs, or videos in a bold way. Its primary menu and widget area are tucked behind a handy button, giving your content plenty of room to breathe, and Isola looks beautiful regardless of the device or screen size.
License: GNU General Public License v2 or later
License URI: http://www.gnu.org/licenses/gpl-2.0.html
Text Domain: isola
Tags: blog, lifestream, photography, clean, white, light, one-column, custom-background, custom-colors, custom-header, custom-menu, rtl-language-support, translation-ready, journal, photoblogging, simple, featured-images, fixed-menu, post-formats, infinite-scroll, fixed-layout, responsive-layout
This theme, like WordPress, is licensed under the GPL.
Use it to make something cool, have fun, and share what you've learned with others.
Isola is based on Underscores http://underscores.me/, (C) 2012-2016 Automattic, Inc.