How can I change the icons used to display filetypes in a file manager

[goemonburo]

@CatKiller, thank you again -- that's much clearer and essentially what I thought needed to happen. But now the mystery: in the icon theme I THINK I'm using (Arc, at least when I see it in Gnome-Tweaks), there already ARE icons that have "application-x-cd-image.png."

So with icons already there...why aren't they showing up?! Is there some additional step I'm missing? Is it the fact that I've changed the default program they're opened with from whatever it was to VLC and there's no corresponding VLC icon in this theme??? Could that be it?

[CatKiller]

So with icons already there...why aren't they showing up?!

No idea. I don't use Lubuntu and I don't use Arc. As far as I can tell, Arc's icon for the cd image MIME type should look like this. Is that what you see? That could look like a "blurry document" at some sizes, I'd imagine. Or you could be using something that isn't Arc.

You can always copy the icon theme's directories to the appropriate place in your Home folder and change any icons that you don't like. Per-user themes override system themes and don't get changed by upgrades.

Changing the default application through the file browser wouldn't have affected anything, but I suppose it's possible that VLC messed with the MIME database in some way to lay claim to those files if you did it through VLC. It shouldn't have, but applications can always do the wrong thing and Windows, where messing with the MIME database could well be an appropriate action, is VLC's primary development target.

Potentially relevant: from the Arc readme

This theme doesn't provide application icons, it needs another icon theme to inherit them. By default this theme will look for the Moka icon theme to get the missing icons. If Moka is not installed it will use the Gnome icon theme as fallback. To change the application icons, edit Arc/index.theme and replace Moka with the name of your preferred icon theme

so the Arc devs suggest using Moka to fill in any gaps in what their own theme provides.

[Dennis N]

@Dennis N, this was exactly what I needed. Thank you.
So when I right click and check properties, I don't get anything that says MIME type. I do have a line that says "file type" and it's "raw cd image." So how do I sleuth out the location of that image?

I installed Arc Icon Theme from gnome-look.org on Lubuntu 19.10 to check it out. I attached the Properties window from an .iso file. Mine shows the MIME type. Also, notice there is a preview of the icon for the file in the upper left (a disk), and that's what shows in my PCManFM file manager as well (2nd screenshot). You don't you get that?

If not, I suggest you re-acquire the icon theme and replace the existing set.

[goemonburo]

Screenshot from 2019-12-02 16-01-00.png

So this is what I'm getting for my .iso file. And if I right click and choose properties, it's this:

Screenshot from 2019-12-02 16-04-25.png

I've read the process for changing themes and it seems straightforward: put it in your .themes or .icons folder, use "gnome-tweaks" (or Unity-tweak-tool?) to select it. Log out, log in...voila.

No?

If I go into the theme, I can see that same image (white page with "cd" circles) in all the various sizes and folders.

Pulling my hair out at this point. It's not even that important, so I may just live with it, but ideally, it just seems like something that should be easy to identify and fix. Right?

[goemonburo]

Also, just in case it isn't obvious -- the mp4 file is just there as an example of one that's working. Most of the other file types seem to be fine, like spreadsheets, text documents, audio and so on...

[goemonburo]

Okay, I believe that I've gotten somewhere:

1) By going through and looking at the icons I've got showing, they appear identical to Adwaita's, NOT Arc, as I'd hoped.
2) Yet when I use gnome-tweaks to choose, I see that Arc is showing up in both the Theme and Icon options.

Does this get me any further??

[CatKiller]

Well, you know how to add extra icons to Adwaita now.

Other desktop environments aren't as limited as Gnome; they don't need a helper application just to set preferences. You aren't using Gnome, so there's no point in using gnome-tweak. Set your theme in your normal Preferences/Customisation area.

[Dennis N]

I've read the process for changing themes and it seems straightforward: put it in your .themes or .icons folder, use "gnome-tweaks" (or Unity-tweak-tool?) to select it. Log out, log in...voila.

No?

Tell us what version of Lubuntu you are using. What I have posted came from Lubuntu 19.10. Also from your post #9, it seems you are using multiple file managers? Don't install gnome tweaks tool, as that will also install gnome desktop environment! When you stray from a standard Lubuntu install (like I am using) by adding other file managers or additional desktop environments, I can't predict what effects that will have. Sorry.

Other comments:
There is no Arc icon theme in the repositories, so you must have downloaded it as an archive file from someplace else? In Lubuntu, changing to the downloaded theme is simple. Just extract the theme folder from the archive, and copy it to one of:

/usr/share/icons
~/.local/share/icons
~/.icons

It should then be available for selection.

Select icon theme from: Menu > Preferences > LXQt Settings > Appearance > Icons Theme
Once selected in this dialog, click "Apply" and it should update the theme instantly.

[goemonburo]

Solved. @CatKiller, you get the bonus points, though I wish we'd started with that original suggestion: change the theme in that Preferences > Customize Look and Feel. That's what I needed. Chose "Arc" and instantly all was good in the hood. I had a feeling it was actually that easy. Just had to use that tool rather than gnome-tweaks or unity-tweak-tool.

I'm seeing what I should be now. Whew!