Fix to improve saving process.

• Asynchronized to prevent crashes.
• Clarified whether the Apply button is enabled or disabled based on the saved results.
• Fixed to display all error messages.
• Added a notification that the apply process will take a long time.
• Fixed the installation process of ubuntu theme which is required first.
• Fixed non-ubuntu derivatives to work.

BUG:454891
BUG:449222
BUG:399901
BUG:383666

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I researched it to work with many distributions.
It turns out that the following command can be used in almost all cases except ubuntu derivatives.

plymouth-set-default-theme -R {theme}

Distribution Name	update-alternatives	initramfs-tools (update-initramfs)	dracut	mkinitcpio	plymouth-set-default-theme -R {theme}	How to change the plymouth theme
KDE Neon (Ubuntu)	default	default				https://askubuntu.com/questions/1399671/how-to-install-plymouth
Debian	default	default			good	https://wiki.debian.org/plymouth#Configuration
Fedora	default		default		good	https://fedoramagazine.org/howto-change-the-plymouth-theme/
OpenSUSE	default		default		good	https://forums.opensuse.org/showthread.php/499090-How-to-chose-Plymouth-theme
Gentoo	default		need install		good	https://wiki.gentoo.org/wiki/Plymouth#Themes
Manjaro (Arch)				default	good	https://wiki.archlinux.org/title/Plymouth#Changing_the_theme

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Improved loading messages and visibility.

Light	Dark