Window > Themes > Save Theme to File will save your customizations as a .scrtheme file. If you subsequently make changes and then want to reload your custom theme, do Window > Themes > Load Theme from File and select your .scrtheme file.
You can also share the .scrtheme with other people. See this thread for an example of a poster here doing this: Scrivener 3.1 Accessibility - #19 by BadScribbler
Finally, if you do Window > Themes > Import Themes and select your .scrtheme file, that will copy it to a special Scrivener Themes folder and make it permanently available from the Window > Themes menu picker.
Best,
Jim