Change Palette/Theme But Preserve Existing Colors

Hi, in Office 365, wanted to customize color palette (or "theme", i still use the word "palette"), however doing so affects some existing cells.

I do not wish to change the color of existing cells.

Appreciate any help.


Additional nonessential information:

I really want to customize the full color palette so going forward, colors I use most commonly are easily available on the palette without additional clicks for 'more colors', etc. But I see Excel has made it virtually impossible to do so, with the user only allowed to select the top color on the palette and Excel determining the remaining colors in each color column based on factors like "80% darker", etc. Frustrating. I downloaded a super neat 150-theme package for free, but I can't use any new palette until I figure out how to easily preserve the existing colors in this fairly large workbook.