Making them mandatory only made sense when we only supported those 3 properties. Now we support many more properties, so these shouldn't be mandatory.
What do you think about this?
Some investigation into this: removing the check that makes the 3 properties mandatory doesn't work well. There is a theme existence check in LightweightThemeConsumer.jsm based on headerURL, and not specifying that makes the check falsy. Also, a lot of lightweight theme CSS is based on the assumption that accentcolor/textcolor/headerURL are always defined.
A very simple fix would be adding: if (!details.images || !details.images.headerURL) { details.images.headerURL = ""; } to ext-theme.js and removing the checks making accentcolor/textcolor/headerURL mandatory. A better fix would be refactoring LightweightThemeConsumer.jsm to stop using `active = !!aData.headerURL` as "theme is active" check, but that's quite complicated to do with the current setup unfortunately.
These properties are mandatory, because we don't want theme authors to create invalid Light-Weight Themes. If you want the properties to become optional, then we need to move this validation to ext-theme.js: 1. If only LWT properties are present in the manifest, kick off validation that none of them is missing. Ex.: When only 'accentcolor' is defined in the manifest, then we should assume that it's trying to 'behave' like a LWT, thus we should fail validation, saying 'textcolor' and 'headerURL' are also required in this case. 2. Exempt the theme.update() function from this requirement & skip the validation step. LWTs are manifest-only (near future, I know), so we needn't impose this restriction to other theme API consumers.
