Allow themes to load stylesheets conditionally if a certain CSS file exists

Sorry -- the example syntax above should read:

stylesheets[all][] = theme.css
stylesheets[print][] = print.css

stylesheets[all][views][] = views.css
stylesheets[print][views][] = views-print.css

Crossposting #474684: Allow themes to declare dependencies on modules.

Alternatively, we could take a cue from Conditional Stylesheets and handle it this way:

stylesheets-module[views][all][] = views.css
stylesheets-module[views][print][] = views-print.css

@Jacine: I'm not sure this is a duplicate. What I'm suggesting is adding stylesheets based on whether a module is enabled -- not overriding the stylesheets provided by a module. The issue you linked to appears to replace CSS files rather than adding another CSS file based on the presence of a module. (Please correct me if I'm wrong on this.)

The use case is this: A theme can "support" a module by modifying the styling of its output. In the example of the Flag module, a theme that "supports" Flag would slightly modify its output -- changing the throbber, turning the link text into something that looks like a button, etc. -- but not completely replace the Flag module's CSS. In other words, I want to keep the module's flag.css file and add my own flag-mytheme.css file on top of it, but only when if the Flag module exists.

Completely replacing the Flag module's CSS -- i.e., copying most of the module's CSS file -- when only a few tweaks are necessary is overkill and places a huge burden on the theme maintainer to sync her CSS with the module's each time the module changes.

Since this overlaps with some existing issues, I created a single issue to coordinate efforts: #1214006: Unified solution to override/remove/amend and conditional stylesheets in a theme's .info file.

@sun: My intention is not necessarily to load an additional stylesheet if a certain stylesheet has already been added. While I can understand the need for specificity when overriding CSS files, the idea here is to add stylesheets when functionality (in the form of modules) is detected on the site. If the Flag module is enabled, the theme will automatically add its own CSS to enhance was the Flag module already provides. If Flag is disabled, the theme will not add that CSS.

I agree that what I'm suggesting could be accomplished using the detection of specific CSS files rather than the module that supplied it, but I feel that approach requires more maintenance on the part of the themer, who would need to update the .info file if the module changed the name of its stylesheets, split them into multiple files, or merged them into fewer files. If this doesn't happen very often -- and if it's not too difficult to figure out which CSS file triggers the addition of another stylesheet -- then I think the stylesheets-additional[] property would be fine.

@sun: Ah, I see. The issue of a module not necessarily being used on a page hadn't occurred to me. In that case, it makes sense to test for the presence of a module using its CSS file(s) rather than module_exists().