Several Theme Fixes - New Preference Format, Color Tweaks

[thsparks]

This contains a few theme fixes.

Changed theme preference storage to be target-based
Fixes microsoft/pxt-arcade#6735
Before, it was a single theme id regardless of target, so if you changed themes in arcade, that would clear your preference for microbit, and so on...

Menu Text Change
Change "Select Theme" to just "Theme" (more consistent with "Language" in the same menu)

A Few Color Fixes

1. Fixes "Arcade Dark": The words 'Prepare Your Ship' cannot display. pxt-arcade#6743
2. Fixes "Arcade Legacy" "Arcade Light": The contents under the simulator can't be visible clearly after switching to JavaScript and Python pxt-arcade#6748
3. Fixes "Arcade Dark": The text can't be clearly visible in the 'Project Saved!' dialog. pxt-arcade#6739
4. Fixes "Arcade Dark" "High Contrast": The hint can't be clearly visible in the JavaScript and Python tutorials. pxt-arcade#6745
5. Fixes Cannot see the category in toolbox when set High Contrast and Green Screen On pxt-arcade#6309
6. A few others that didn't have bugs filed... (disabled buttons, new/cloud project cards hover, neutral labels, fg/bg mismatch on home page categories)

Additional Tweaks
Some additional changes & refactoring based on findings while working on theme builder, including:

1. Adding a neutral base color
2. Specifying setColorThemeById instead of just setColorTheme, leaving the door open for setting by theme object in the future
3. Making it optional to save the preference when setting the theme
4. Partial changes toward fixing Need Dark-Theme Versions of Dark Icons pxt-arcade#6751 but also need arcade changes for that.

Upload Target: https://arcade.makecode.com/app/7d7ad2f89b8743f9b1e89290dfb6a8d17e39db85-d113d4beed