As with all mods that change GUI, it looks kind of bad with resource packs.
I believe the "dark mode" that a couple of vanilla style resource packs (e.g. Vanilla Tweaks) use and the GUI changing mods that support one, all use the same grey tones.
It would be nice if you added an option in the Mod Menu for enabling a dark mode that changes the mod's GUI to these same colors. It would be applied to all three GUI windows, the latter two would then have a dark grey border instead of the vanilla white/light grey one, the normal inventory GUI window would just have to be adjusted.
Here's a screenshot of the inventory GUI for reference:

These are the hex codes for the grey tones to save you some time if you decide to do it:
Light grey: 6b6b6b
Medium grey: 535353
Dark grey: 333333

Hi there,
I like this suggestion, it's easy to implement so I'll include it in the next release.
Thanks.

Hi there,
I've made assets for dark mode available here. Since there are many different "dark mode" gui resource packs, I've used the colours specific to Vanilla Tweaks > GUI > Dark UI pack.

You can include the aforementioned assets in a resource pack to have PlayerEx in dark mode. This makes more sense than implementing extra textures in the mod and another config option.
Thanks.

@CleverNucleus hey! the gui.png asset you mentioned above is missing the buttons and icons

There i fixed it :)

Hi there @Balakay420,
Thanks for catching that for me, I've now uploaded the fixed gui.png asset.

Edit: or not, it's taking a while to see the changes on the repo.