Re-theme QtGraphs

QtGraphs (https://doc-snapshots.qt.io/qt6-dev/qtgraphs-index.html) is a module to replace QtCharts (https://doc-snapshots.qt.io/qt6-dev/qtcharts-index.html) and QtDataVisualization (https://doc-snapshots.qt.io/qt6-dev/qtdatavisualization-index.html).

Both QtCharts and QtDataVisualization have built-in themes for color for the following attributes:

• window
• background
• background grid
• label background
• label border
• label font
• series x 5 (3D) / ? (2D) (for example bars / lines / scatter point sets) - number can be changed to whatever seems good by UX standpoint. When there are more series' than colors, the colors are re-used from the beginning. The same number of predefined colors should be used for both 3D and 2D in the new design, perhaps?
• selection highlight
• multi-selection highlight

Additionally, the following can be defined by theme:

• label background on/off
• label border on/off
• font family & size
• series coloring is uniform or gradient (gradient is automatically created from uniform color, so no need to define them separately)
• background on/off
• background grid on/off

The themes in use were defined over 10 years ago, so they are outdated and should be brought to modern ages and branding.

Currently there are 7 predefined themes (https://doc-snapshots.qt.io/qt6-dev/q3dtheme.html#Theme-enum), but that number can be changed to anything that is deemed necessary.