You will have to take in to account a couple of things:
The new accent colour enabled stylesheet
Making the app just as usable when the dark style is activated
Maybe dropping the current solution for round corners in favour of Hdy.Window so that it makes styling a bit easier
You should probably use the GitHub projects feature to manage the changes, since there are a bunch of deprications too:
[5/29] Compiling Vala source src/constants.vala .....vala ../src/shortcuts.vala ../src/xmlHandler.vala.
../src/bookworm.vala:29.5-29.53: warning: `Granite.Services.Paths' has been deprecated since 5.5.0. Use GLib.Environment
../src/bookworm.vala:37.5-37.40: warning: BookwormApp.Bookworm.options hides inherited field `Granite.Application.options'. Use the `new' keyword if hiding was intentional
private static OptionEntry[] options;
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
../src/shortcuts.vala:659.13-659.20: warning: `string.size' is deprecated. Use string.length
../src/bookworm.vala:125.28-125.49: warning: `Granite.Services.Paths' has been deprecated since 5.5.0. Use GLib.Environment
../src/bookworm.vala:126.9-126.31: warning: Access to static member `Granite.Services.Paths.initialize' with an instance reference
app_xdg_path.initialize (Constants.bookworm_id, Constants.INSTALL_SCRIPTS_DIR);
^^^^^^^^^^^^^^^^^^^^^^^
../src/bookworm.vala:126.9-126.31: warning: `Granite.Services.Paths' has been deprecated since 5.5.0. Use GLib.Environment
../src/bookworm.vala:127.32-127.60: warning: Access to static member `Granite.Services.Paths.user_data_folder' with an instance reference
bookworm_config_path = app_xdg_path.user_data_folder.get_path ();
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
../src/bookworm.vala:127.32-127.60: warning: `Granite.Services.Paths' has been deprecated since 5.5.0. Use GLib.Environment
../src/window.vala:182.21-182.43: warning: implicit .begin is deprecated
../src/contentHandler.vala:182.13-183.67: warning: unhandled error `GLib.Error'
../src/dialog.vala:296.9-296.32: warning: `Gtk.FontButton.set_font_name' has been deprecated since 3.22
../src/dialog.vala:506.9-506.41: warning: `Gtk.ScrolledWindow.add_with_viewport' has been deprecated since 3.8
../src/library.vala:679.13-679.55: warning: implicit .begin is deprecated
../src/dialog.vala:581.42-581.65: warning: `Gtk.FontButton.get_font_name' has been deprecated since 3.22
../src/dialog.vala:820.13-820.36: warning: `Gtk.FontButton.set_font_name' has been deprecated since 3.22
../src/fb2Reader.vala:210.17-210.60: warning: unhandled error `GLib.Error'
Pixbuf pixbuf = new Pixbuf.from_stream (mis);
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
../src/contentHandler.vala:414.13-415.68: warning: unhandled error `GLib.Error'
../src/window.vala:339.13-339.49: warning: implicit .begin is deprecated
../src/window.vala:511.29-511.38: warning: `string.len' is deprecated. Use string.length
../src/window.vala:674.17-674.53: warning: implicit .begin is deprecated
../src/headerbar.vala:138.17-138.54: warning: implicit .begin is deprecated
../src/bookworm.vala:241.17-241.29: warning: implicit .begin is deprecated
../src/bookworm.vala:274.13-274.49: warning: implicit .begin is deprecated
../src/main.vala:33.13-33.49: warning: Access to static member `BookwormApp.Bookworm.command_line_option_debug' with an instance reference
application.command_line_option_debug = true;
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
../src/main.vala:36.13-36.48: warning: Access to static member `BookwormApp.Bookworm.command_line_option_info' with an instance reference
application.command_line_option_info = true;
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
../src/dialog.vala:1104.13-1104.25: warning: local variable `shortcutValue' declared but never used
var shortcutValue = "";
^^^^^^^^^^^^^
../src/contentHandler.vala:28.14-28.29: warning: local variable `shouldReloadPage' declared but never used
bool shouldReloadPage = true;
^^^^^^^^^^^^^^^^
../src/shortcuts.vala:451.41-451.49: warning: copying delegates is not supported
../src/shortcuts.vala:461.39-461.47: warning: copying delegates is not supported
Command-line option `--thread` is deprecated and will be ignored
Compilation succeeded - 29 warning(s)
You will have to take in to account a couple of things:
You should probably use the GitHub projects feature to manage the changes, since there are a bunch of deprications too: