To start, hover over the content block you want to edit and click the Settings icon. Then under the 'Background' tab, you'll be able to select the new background color.
Underneath that, you'll see a little checkbox that says 'Apply to all with same colour'. Tick the checkbox for this and you can also check the option for any applied filters and then click 'Save and Preview'.
Once saved, all the content blocks inside your document or template that are that exact same colour previously will all have been updated to the new colour you selected.