Supported SMF Versions
This version supports the most recent SMF 2.0.X & SMF 2.1.X branches.

Recommended Minimal Requirements
OS minimum recommended versions: Ubuntu 18, Centos 7
HTTP Server: Apache 2.4 using PHP 7.1
Database: MySQL / Maria DB (using UTF-8 Unicode)
Extensions: curl, mbstring, zlib, phar
Forum software/versions: SMF 2.0.17 or SMF 2.1.0 ~ (using UTF-8 Charset)
Hypertext Transfer Protocol Secure (HTTPS) ~ HTTP over TLS or HTTP over SSL

RAR support is only optional & not required
Optional RAR compression support: RAR (command-line tool) -> shell_exec must be enabled in PHP to use RAR compression
Optional RAR decompression support: UNRAR (PECL RarArchive class) package for Unix -> Extension must be enabled in php.ini (check phpinfo() to confirm)
RAR archive package for PHP 7.3 (check other PHP versions): https://centos.pkgs.org/8/remi-x86_64/php73-php-pecl-rar-4.0.0-4.el8.remi.x86_64.rpm.html
RAR compression for Unix: Install rar via yum, DNF or apt-get -> example: yum install rar
RAR compression for WIndows: Requires WinRAR to be installed in -> Program Files / Program Files (x86)

This modification requires an updated version of PHP to at least v7.1.
You likely are using an outdated version of PHP or do not have one of the required PHP modules installed.

Oops.. my bad.

Well it appears I overlooked one setting in the forum to allow users to change the theme.
I previously looked for it in user group and permissions where I did not see a setting for it.
It was on the theme admin page as a checkbox to allow all users to opt their own theme.
That was likely the problem. It should work now.

Ok, I added some JQuery to submit the form when a drop-down theme is selected.
Does it work for others now?

I will make some changes later tonight by attempting to use JQuery instead of vanilla JavaScript.
Perhaps that will work.

It is puzzling that it works for me without issue but 3 people here say otherwise.

Strange because I have tested it using Chrome plus Edge on Windows 10 and it works fine for me.
I also tested it on an Android device with the default browser and it works fine for me.

That looks like Z-A for desktop/laptop.
Also keep in mind that the arcade remembers the game type selected and saves it in your user profile.
It will remember some of those settings for the next time you use the arcade.
This can be reset by clicking on the Arcade button/link located at the bottom left of the top skin.
The default sorting for the arcade (when reset) is latest descending order.

To test it, navigate to the arcade here on the site & click on the arcade link described above.
Then afterward click on A-Z (not Z-A) from the drop-down.

Yes there was a known problem with the sorting order in the current stable release.
The next release has a fix that should allow it to work properly.
Test out sorting games in the arcade here on this site & let me know if you think it seems to be working now.

Try it out now & let me know if it works.

I enabled the theme selection block at the top of the forum.
If you look closely there is a small drop-down with the available themes.

I suggest the Waverider theme which is the one that used to be active by default.
It is set to save your theme option so your opted theme will stay in effect until you change it again.

Since you are familiar with editing files, I will assume you know what to do with the attachments.

It seems to be working now.
Are the errors not showing at the moment?

When attempting to edit files from modifications for the purpose of personalization or translating to your community native language, make sure you use a proper editor that is designed to use the proper encoding & EOL's.

Many people attempt to use Microsoft's notepad which will cause errors and/or likely cause your modification or forum to crash.

To edit files from SMF or any of its modification's I will suggest that you use Notepad++ (Notepad-plus-plus).
It is a free third party PC application that has no ads & no gimmicks.
Imo you should make it your default application to edit txt, php, js, & css files.

You can download this PC application here:
https://notepad-plus-plus.org/downloads/

Your website is down at the moment.
I don't get this error here on the arcade nor on other sites using SMF 2.0.

Also note that you mentioned recently adjusting session settings in a previous post.
You may have something not set properly.

---

Past conversations that may or may not be relevant:
Also we previously talked about forums in sub-directories of your main forum can cause session errors.
Have you recently been running other forum software in sub-directories (test sites or otherwise)?
If you want multiple forums it's best to have them run off of the main public_html directory then set up your DNS & Apache settings to direct the connections to those directory paths.

That's the signup page.
The arcade has a custom mobile template for the login page but might need one for the signup page.
I haven't looked at that part for quite some time but will look at it now to see what can be done.