Fix white ApexCharts backgrounds under dark themes (deepness) by setting chart.background from the theme

[TheWitness]

Summary

PR #53 skinned the ApexCharts views from the active Cacti theme's rrdtheme.php,
but only wired up the text (foreColor), gridlines (gridColor), and the
donut track. It never set the ApexCharts chart.background, so ApexCharts kept
painting its default white plot surface regardless of the computed
theme.mode. Under any dark theme (e.g. deepness) the By Method / By Table bar
and box charts — and the import upload donut — still rendered as white boxes on a
dark page.

Details

• slowlog_apex_theme() now also resolves the active theme's own panel
  background: the rrdtheme.php back color, falling back to the plotting
  canvas, then to white for light-theme defaults. It is normalized the same way
  as the font/grid colors and returned as a new background key.
• That background is applied as chart.background on all three charts: the bar
  chart (renderChart), the box chart (renderBoxChart), and the upload donut
  (initDonut).
• For deepness, this resolves to #11161d (its back), so the charts now match
  the surrounding dark panels instead of showing white.

No behavioral change to data, imports, or chart contents — colors only.

Testing

• php -l clean on includes/functions.php, slowlog.php, and the test file.
• tests/Unit/ApexThemeTest.php extended: back added to the dark fixture, a new
  canvas-only fixture to cover the back→canvas fallback, and background
  assertions on the light, dark, canvas-fallback, and missing-theme cases.

[Copilot]

🟢 Approval recommended

The focused implementation is consistent across all chart types and adequately covered by existing theme tests.

0 open findings

What changed in this PR

Adds theme-aware ApexCharts backgrounds so charts blend correctly with dark Cacti themes.

Changes:

• Resolves chart backgrounds from back, then canvas, then white.
• Applies the background to bar, box, and upload donut charts.
• Extends theme tests and documents the fix.

File	Description
includes/​functions.php	Adds normalized background color resolution.
slowlog.php	Applies the background to all ApexCharts instances.
tests/​Unit/​ApexThemeTest.php	Tests theme backgrounds and fallback behavior.
CHANGELOG.md	Records the dark-theme rendering fix.

🧠 Review effort: Balanced

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