Thanks Jim. I did chance upon the "Save as Template" right-click option. But i wasn't able to apply it to my new graphs, or set it as a default for the file. Any ideas on how to use it? Thanks again for the response. And now you
mention it, I do recall trying to set a colour palette in PowerPoint some years ago but struggled to get it into Excel IIRC.
Fast forward to now on latest version of PowerPoint 365 for macOS and there is no option to create a customized colour palette, not for me anyhow.
Here's what the popup Help file says:

And here is what I see when I click on the little arrow below the theme selection panel:

As you can see, no colour options at all.
EDIT ::
… typical MS Office nonsense, there are two different boxes that look like one box and the relevant arrow is under the other right hand theme box. Who knew… hope this saves someone else a few hours in the future trying to navigate the rock bottom garbage MS
UI concepts. How do they manage to keep going from bad to worse. The OneDrive UI/US is a text-book case of breaking more UI principles in one implementation than you can poke a stick at. The entire ribbon concept could be consigned to the garbage heap for
floating and/or docked palettes like mature software interfaces. The Ribbon info hierarchy and UI is confusing as hell I think, one often needs to access 3 or more ribbon tabs in a row to do a simple thing to one element in the sheet.
Roadblock: this awkward method to save a colour set for graphs limits one to six colours for graphs (4 are assigned to text/background and 2 to hyperlinks). Great work MS not only is this method frustrating, it doesn't even work for more than 6 chart colours.
Even better, while the colours are immediately available in Excel without a restart of Excel (good), they aren't available as chart colours directly, you still have to make a theme, but limited to six colours so you may as well just use the macOS colour
inkwell dock on the macOS colour picker. And when i loaded the theme, it didn't change the colours of the last graph I manually hand coloured and but
did change the colours of several other graphs. Basically, I hate the entire concept of themes — they don't work for graphs. I need multiple themes and I'm limited to one per sheet or per file (not sure which yet).
I hope I work out how to load a chart template into a new graph… it's the only hope left that I can automate this process inside Excel. As I'm a designer with Adobe Illustrator it's my clear best option is the one I've been using
for 10 years, print the graphs to individual PDF files, on per graph. Open in AI, remove a load of clipping paths and other empty boxes and gunk and hand colour using efficient tools made for professionals whose time is valuable.
