custom deck_backgrounds naming issue

[yakaru]

Before posting, please verify that you've done the following

• read the "before you install instructions" and disabled all the known conflicted add-ons.
• You've looked through previous issues to see if someone else had the same problem.

Describe the bug
The code that sets an individual deck background seems to be using the string:
'{base}/user_files/assets/deck_backgrounds/%(deck)s.jpg' (deck_overview.py around line 410)
the issue is that subdecks are delineated by :: and in Windows : is an invalid filename character, so you are unable to set custom backgrounds for any subdecks you use (nor does the root deck apply to the subdeck, which might be more a feature request than a bug)

Screenshots
I could provide screenshots, though the bug is easy to reproduce

Contex (please complete the following information):

• OS: Windows 10
• Anki Version 25.02
• screen Resolution 1920x1080

Additional context
In your Anki
go to help > About > Copy Debug Log and paste it between the following backticks

Anki 25.02 (038d85b1)  (ao)
Python 3.9.18 Qt 6.6.2 PyQt 6.6.1
Platform: Windows-10-10.0.19045

===Add-ons (active)===
(add-on provided name [Add-on folder, installed at, version, is config changed])
AnkiConnect ['2055492159', 2025-02-25T14:57, 'None', '']
AnkiWebView Inspector ['31746032', 2023-06-27T12:26, 'None', '']
Beautify-Anki 20 mizmu ['1116770498', 2023-10-31T11:51, 'None', mod]
Kanji Grid Kuuube ['1610304449', 2025-03-26T05:08, 'None', '']
Migaku Anki Add-on ['1846879528', 2025-03-13T01:29, 'None', mod]
Review Heatmap ['1771074083', 2022-06-29T18:43, 'None', '']
Straight Reward ['straight_reward', 0, 'None', '']
True Retention ['613684242', 2017-11-19T12:43, 'None', '']
anki-forvo-dl - Forvo Pronunciation Downloader fully automatic ['858591644', 2024-08-09T19:15, 'None', '']

===IDs of active AnkiWeb add-ons===
1116770498 1610304449 1771074083 1846879528 2055492159 31746032 613684242 858591644

===Add-ons (inactive)===
(add-on provided name [Add-on folder, installed at, version, is config changed])