feat: Use system theme

[WalhallaZ]

Motivation:
Allow users (myself) not to lose eyesight when opening a light app in a dark themed OS.

Changes:

1. Update to .NET 10 (.NET 9 could have worked, but didn't test with it) to allow using native windows theming capabilities + auto switch between light/dark
2. Kill all hardcoded brushes
3. Implement custom datetime picker to avoid theme issues with xceed control - dates and times become unreadable there. Couldn't find a workaround.
   3.1. CheckComboBox is still used - despite not respecting dark theme it's still readable.
4. Adjust control sizes while still preserving compact look & feel of the main datagrid

Important:
Only tested on Win 11 25H2 (build 26200.7171). I don't know what OS versions you try to support.

Screenshots dark:

Screenshots light:

Space for improvements (either find a way to override xceed styles or make a custom control):

[kmaki565]

Thank you for the very interesting PR! I didn't know WPF natively supports Fluent now.
I see many UI issues so I cannot merge this into the main branch now, but maybe we can continue developing this in a develop branch so we can adopt it in the future.

Issues which I think need to be addressed:

• Unchecking truncate button does not show full message text (we should remove the fixed height in DataGrid Row)
• Expander is ugly and not easy to operate. Maybe we'd better remove it.
• CheckComboBox does not follow Dark mode.
• Can everything be compact? I want to adjust menu spacing, font size, and so forth.
• Check compatibility. As I tested on a Windows 10 PC, it did not display correctly in Light mode.
• The title and status bars need to be Fluent.
• The menu bar needs to be inside the title bar to yield more space for DataGrid.

The last one may need a relatively big surgery, but we could refer to WPF Gallery's source code.

Would it be possible for you to help?

[WalhallaZ]

Hi, sorry for late reply, yes I can take a look at this but it'll probably happen over the weekend, not earlier.

[kmaki565]

Hi @WalhallaZ,
I created a new branch based on your code: https://github.com/kmaki565/EventLook/tree/feature-net10-modern-ui
Let's put our code into this branch!
I'm not very sure if you can reuse this PR, but please target the new branch hereafter.