When working with themes and child themes, you need a plain text editor to view, edit and save theme files. Files as the style.css and the functions.php.
Do not use a word processor like Microsoft Word or LibreOffice Writer. These programs might save the theme files with additional codes regarding markup and meta data that web servers and browsers simply do not understand.
An almost ubiquitous, but simple, plain text editor is Windows Notepad. One of the disadvantages of Windows Notepad however, it that it tends to append ‘.txt’ to file names when saving your work. You can avoid this by putting the file name between quotation marks upon save, like this “style.css”
But do yourself a favor and get a plain text editor. These editors are developed for professional and occasional coders alike, and offer handy features like syntax highlighting. A feature that Windows Notepad lacks.
With syntax highlighting, code elements are displayed in distinguishing colors, which increases the readability.
Syntax highlighting also helps identifying errors. When an code error occurs, the code will be displayed in a different color than expected. As soon as you have corrected (debugged) the error, all code will be displayed in the proper color scheme again.
For more than two years already, I am using Sublime Text 3. Sublime Text is available for Windows, Linux and Mac. The downside is that Sublime Text is not free. It costs a whopping $70. Not cheap, even though it is a one time fee.
Fortunately, there are enough alternatives. Even free. A good and free editor for Windows is Notepad++.
In case your are on Linux, you can use gedit or Kate, or whatever editor comes with your Linux desktop. You can even use vi, Emacs or nano.
As soon as the plain editor of your choice is installed, you are ready to work with WordPress theme files.