Microsoft's Visual Studio Code (https://code.visualstudio.com ticks all these boxes. It includes a Centered Layout (content is positioned in the middle of the editing panel -- with ability to expand / contract the width of the content), and a Zen Mode, which is full-screen, no toolbars or anything on the screen except the text you are writing. Centered Layout and Zen Mode can be combined.

"Tree view" - VSCode does this with as many different folders you want at a time.
"Bold text" -- when using markdown in preview mode
"White background" -- white, or any theme you want
"text is centered" -- as I mentioned above
"basically nothing else on the screen" -- as I mentioned above

VSCode is available on Mac and Windows. It is enormously configurable and can be overwhelming. But is also easily configurable into a simple editor. And best of all -- it is FREE!

doablesoftware wrote:

1. tree view of folders/files on the side

2. **able to bold the text so you can see the bolded text**

3. white background

4. text is centered on the page/typing area

these 4 basic things needed

--

this one is a plus but not absolutely needed,

* basically nothing else on the screen (clean visually minimal UI)