Use the regular preprocess-function for the CSS files as well

An old shortcoming of the `preprocessCSS`-function is its complete lack of support for our "normal" defines, which makes it very difficult to have build-specific CSS rules. Recently we've started using specially crafted comments to remove CSS rules from the MOZCENTRAL build, but (ab)using the `preprocessCSS`-function in this way really doesn't feel great.
However, it turns out to be surprisingly simple to instead use the "regular" `preprocess`-function for the CSS files as well. The only special-handling that's still necessary is the helper-function for dealing with CSS-imports, but apart from that everything seems to just work.

One reason, as far as I can tell, for having a separate `preprocessCSS`-function was likely that we originally used *lots* of vendor-prefixed CSS rules in our CSS files. With improvements over the years, especially thanks to Autoprefixer and PostCSS, we've been able to remove *almost* all non-standard CSS rules and the need for special-casing the CSS parsing has mostly vanished.

*Please note:* As part of testing this patch I've diffed the output of `gulp generic`, `gulp mozcentral`, and `gulp chromium` against the `master`-branch to check that there was no obvious breakage.

web/viewer.css

@@ -29,7 +29,9 @@
 
   --main-color: rgba(12, 12, 13, 1);
   --body-bg-color: rgba(237, 237, 240, 1);
-  --errorWrapper-bg-color: rgba(255, 110, 110, 1); /* -webkit-non-mozcentral */
+  /*#if !MOZCENTRAL*/
+  --errorWrapper-bg-color: rgba(255, 110, 110, 1);
+  /*#endif*/
   --progressBar-color: rgba(10, 132, 255, 1);
   --progressBar-indeterminate-bg-color: rgba(221, 221, 222, 1);
   --progressBar-indeterminate-blend-color: rgba(116, 177, 239, 1);
@@ -77,7 +79,9 @@
   --toolbarButton-zoomIn-icon: url(images/toolbarButton-zoomIn.svg);
   --toolbarButton-presentationMode-icon: url(images/toolbarButton-presentationMode.svg);
   --toolbarButton-print-icon: url(images/toolbarButton-print.svg);
-  --toolbarButton-openFile-icon: url(images/toolbarButton-openFile.svg); /* -webkit-non-mozcentral */
+  /*#if GENERIC*/
+  --toolbarButton-openFile-icon: url(images/toolbarButton-openFile.svg);
+  /*#endif*/
   --toolbarButton-download-icon: url(images/toolbarButton-download.svg);
   --toolbarButton-bookmark-icon: url(images/toolbarButton-bookmark.svg);
   --toolbarButton-viewThumbnail-icon: url(images/toolbarButton-viewThumbnail.svg);
@@ -112,7 +116,9 @@
   :root {
     --main-color: rgba(249, 249, 250, 1);
     --body-bg-color: rgba(42, 42, 46, 1);
-    --errorWrapper-bg-color: rgba(169, 14, 14, 1); /* -webkit-non-mozcentral */
+    /*#if !MOZCENTRAL*/
+    --errorWrapper-bg-color: rgba(169, 14, 14, 1);
+    /*#endif*/
     --progressBar-color: rgba(0, 96, 223, 1);
     --progressBar-indeterminate-bg-color: rgba(40, 40, 43, 1);
     --progressBar-indeterminate-blend-color: rgba(20, 68, 133, 1);
@@ -457,9 +463,11 @@ select {
 #findInput {
   width: 200px;
 }
+/*#if !MOZCENTRAL*/
 #findInput::-webkit-input-placeholder {
   color: rgba(191, 191, 191, 1);
 }
+/*#endif*/
 #findInput::placeholder {
   font-style: normal;
 }
@@ -817,10 +825,12 @@ select {
   mask-image: var(--toolbarButton-print-icon);
 }
 
-#openFile::before, /* -webkit-non-mozcentral */
-#secondaryOpenFile::before /* -webkit-non-mozcentral */ {
+/*#if GENERIC*/
+#openFile::before,
+#secondaryOpenFile::before {
   mask-image: var(--toolbarButton-openFile-icon);
 }
+/*#endif*/
 
 #download::before,
 #secondaryDownload::before {
@@ -1002,10 +1012,11 @@ a.secondaryToolbarButton[href="#"] {
   background-repeat: no-repeat;
   background-position: 3px;
 }
-
+/*#if !MOZCENTRAL*/
 #pageNumber::-webkit-inner-spin-button {
   -webkit-appearance: none;
 }
+/*#endif*/
 
 .toolbarField:focus {
   border-color: #0a84ff;
@@ -1174,7 +1185,8 @@ a:focus > .thumbnail > .thumbnailSelectionRing,
   background: rgba(0, 0, 255, 0.3);
 }
 
-#errorWrapper /* -webkit-non-mozcentral */ {
+/*#if !MOZCENTRAL*/
+#errorWrapper {
   background-color: var(--errorWrapper-bg-color);
   color: var(--main-color);
   left: 0;
@@ -1184,17 +1196,17 @@ a:focus > .thumbnail > .thumbnailSelectionRing,
   padding: 3px 6px;
 }
 
-#errorMessageLeft /* -webkit-non-mozcentral */ {
+#errorMessageLeft {
   float: left;
 }
-#errorMessageRight /* -webkit-non-mozcentral */ {
+#errorMessageRight {
   float: right;
 }
 
-#errorSpacer /* -webkit-non-mozcentral */ {
+#errorSpacer {
   clear: both;
 }
-#errorMoreInfo /* -webkit-non-mozcentral */ {
+#errorMoreInfo {
   background-color: var(--field-bg-color);
   color: var(--field-color);
   border: 1px solid var(--field-border-color);
@@ -1202,6 +1214,7 @@ a:focus > .thumbnail > .thumbnailSelectionRing,
   margin: 3px;
   width: 98%;
 }
+/*#endif*/
 
 .dialogButton {
   width: auto;