If this requires write access, we might consider treating the results as cache, and put them somewhere under ./data/
We have ext.php to serve files that are not directly exposed publicly, which we could generalise.

Sure, that makes sense.

@Alkarex Unfortunately I just noticed a problem.

Ansum, for example, contains things like this:

background-image: url(icons/magnifier.svg);

The less elegant approach of just generating it locally and committing it all would be an easy way around that, but of course I wrote this PR to avoid such a thing. It's an action you can forget and it adds extra noise in commits/diffs.

One possible approach would be to copy the whole thing, along these lines (except in PHP instead of shell for Windows compatibility):

diff --git a/app/FreshRSS.php b/app/FreshRSS.php
index 14995d5e..4ca026e1 100644
--- a/app/FreshRSS.php
+++ b/app/FreshRSS.php
@@ -104,23 +104,27 @@ class FreshRSS extends Minz_FrontController {
                                        $filename_ltr = $file;
                                }
                                $filename_rtl = $filename_ltr . '.rtl.css';
-                               $path_ltr = PUBLIC_PATH . '/themes/' . $theme_id . '/' . $filename_ltr;
-                               $path_rtl = DATA_PATH . '/rtl/'. $theme_id . '.' . $filename_rtl;
+                               $path_ltr = PUBLIC_PATH . '/themes/' . $theme_id;
+                               $path_rtl = DATA_PATH . '/rtl/'. $theme_id;
+                               $path_full_ltr = $path_ltr . '/' . $filename_ltr;
+                               $path_full_rtl = $path_rtl . '/' . $filename_rtl;
 
-                               $filetime = @filemtime($path_ltr);
+                               $filetime = @filemtime($path_full_ltr);
                                $url = '/themes/' . $theme_id . '/' . $filename_ltr . '?' . $filetime;
 
                                if (_t('gen.dir') === 'rtl') {
-                                       if (!file_exists($path_rtl) || filemtime($path_rtl) < $filetime) {
+                                       if (!file_exists($path_full_rtl) || filemtime($path_full_rtl) < $filetime) {
+                                               `rm -rf $path_full_rtl`;
+                                               `cp -r $path_ltr $path_rtl`;
                                                // generate RTL CSS
                                                if (!class_exists('CSSJanus')) {
                                                        require_once LIB_PATH . '/lib_CSSJanus.php';
                                                }
-                                               $content = file_get_contents($path_ltr);
+                                               $content = file_get_contents($path_full_ltr);
                                                $rtl_content = CSSJanus::transform($content);
-                                               file_put_contents($path_rtl, $rtl_content);
+                                               file_put_contents($path_full_rtl, $rtl_content);
                                        }
-                                       $url = '/ext.php?f='. $theme_id . '.' . $filename_rtl . '&t=css&c=rtl&' . $filetime;
+                                       $url = '/ext.php?f='. $theme_id . '/' . $filename_rtl . '&t=css&c=rtl&' . $filetime;
                                }
 
                                Minz_View::prependStyle(Minz_Url::display($url));

That principle works fine, setting aside that the icon URLs still don't actually work at this point.

I looked around a bit and this PR simply puts them in the theme directory, but I figure having those writable isn't exactly desirable.

Also pinging @aledeg @marienfressinaud for their thoughts.

There are two things that bother me.

The first one is to put CSS theme files under the ./data folder (actually, there are a lot of files that I would not put in this folder, it's another non-critical subject). I was even considering dropping the ext.php and f.php mechanisms by replacing them by a dynamic system of "assets collection", to move all CSS and JS under the /p folder and to allow pre-processing. It's a long-term idea that I will not develop soon, but just to say that I don't mind if the server write files in the public folder: I find it's more logical.

The second thing is to do that in loadStylesAndScripts. Writing these files is a huge side-effect of the method. I would have done it at least in a different method of this class (and yet I don't think it's the ideal solution, but I can't find one for the moment ^^)

Last thing: I would like to have the feedback of someone who is used to RTL interfaces. I find the interfaces look pretty weird (but hey, it's normal since I never use RTL interfaces ^^) and I'm wondering if the solution is to simply reverse the rules or if there are trickier things to think about.

A note regarding were to place files: key aspects include whether the folder is writeable by the Web server, and whether the whole folder can be erased and replaced when there is a new FreshRSS version.

Oh right, I see what I wrote, haha. My apologies. I meant this:

Minor details like the arrows (which are in the wrong direction)
and
some missing UI elements.

@WaseemAlkurdi Btw, you can see for yourself over on https://demo.freshrss.org/ if you're curious.

@Frenzie I've taken a look. Very awesome project! I'm thinking of deploying it sometime.
You're correct: the bottom (previous/next) arrows are indeed reversed. Other than that, no issues at all that I can see in the screenshots above vs. the demo.

Edit: Missing UI elements? I can't see any in the screenshots above.

@WaseemAlkurdi I didn't see your edit. It's not the UI elements that are missing per se, but the icons on them (on the second and fourth screenshot).

@Frenzie I had to stare at the screenshots for a couple minutes to even know what was missing! :-)
But true, the icons are missing (the toolbar and search bar, as far as I can see).