Fix email buttons for white theme

[MorrisJobke]

If the theming app has set the color set to white the button in the emails where white with white text. This fixes that and introduces the getTextColorPrimary to the defaults interface to make the color calculation consistent.

[MorrisJobke]

cc @nextcloud/theming

[MorrisJobke]

We maybe should backport this to stable12.

[rullzer]

Failing tests

[MorrisJobke]

Failing tests

Fixed.

[codecov]

Codecov Report

Merging #7427 into master will decrease coverage by 44.38%.
The diff coverage is 0%.

@@ Coverage Diff @@## master #7427 +/- ##
============================================
- Coverage 50.97% 6.59% -44.39% - Complexity 24735 24738 +3 
============================================
Files 1586 1586 Lines 94257 94265 +8 Branches 1365 1365 ============================================
- Hits 48049 6215 -41834 - Misses 46208 88050 +41842

Impacted Files	Coverage Δ	Complexity Δ
apps/theming/lib/ThemingDefaults.php	0% <0%> (-91.43%)	48 <2> (+1)
lib/private/legacy/defaults.php	0% <0%> (-49.04%)	46 <2> (+2)
apps/theming/lib/Capabilities.php	0% <0%> (-81.82%)	3 <0> (-1)
lib/private/Mail/EMailTemplate.php	0% <0%> (-59.85%)	43 <0> (ø)
lib/public/Defaults.php	0% <0%> (-15.79%)	20 <1> (+1)
...eware/Security/Exceptions/NotLoggedInException.php	0% <0%> (-100%)	1% <0%> (ø)
apps/encryption/lib/Session.php	0% <0%> (-100%)	21% <0%> (ø)
apps/dav/lib/Files/Sharing/FilesDropPlugin.php	0% <0%> (-100%)	6% <0%> (ø)
lib/private/Diagnostics/EventLogger.php	0% <0%> (-100%)	9% <0%> (ø)
lib/private/BackgroundJob/QueuedJob.php	0% <0%> (-100%)	1% <0%> (ø)
... and 853 more