allow access to GetSoftenedColor for library users

[wolframhaussig]

About this PR

When writing a theme plugin, we may want to use a slightly different color while still adhering to the general style (e.g. coloring the lines in a table). Before this PR, AbstractTheme.GetSoftenedColor was only available internally and for custom theme implementations. By making it public, plugin writers can now use it to color the table in the theme.TableCellBackColor and alternating rows in theme.GetSoftenedColor(theme.TableCellBackColor)

ToDos

• We should discuss if we want to keep the method in AbstractTheme or not.

Checklist

• I have added a suitable test
• I have updated the documentation in the readme

[github-actions]

File	Coverage		Missing
All files	84%	✅
ITheme.cs	0%	❌	63
ThemeConfig/ThemeRegistry.cs	81%	✅	93-95 101-103
ThemeConfig/ThemeRegistryBuilder.cs	94%	✅	89-90
ThemeConfig/ThemeRegistryBuilder.cs	92%	✅	242-245
ThemeLookup/FileThemeLookup.cs	88%	✅	44-46
ThemeLookup/IThemeLookup.cs	0%	❌	24
ThemeLookup/ResourceThemeLookup.cs	91%	✅	95-97 123-124
ThemeRegistryHolder.cs	75%	❌	19
Utilities/DarkWindowsTheme.cs	80%	✅	32 54
Utilities/WindowsThemeDetector.cs	65%	❌	17-18
Themes/AbstractTheme.cs	91%	✅	236-240 356-358 367-369
Themes/AbstractThemePlugin.cs	100%	✅
Themes/DefaultDarkTheme.cs	100%	✅
Themes/DefaultLightTheme.cs	100%	✅
Themes/FileTheme.cs	79%	❌
Themes/HighContrastDarkTheme.cs	100%	✅
Themes/ToolStrip/ThemedColorTable.cs	100%	✅
Themes/ToolStrip/ThemedToolStripRenderer.cs	85%	✅
Extensions/ColorExtensions.cs	100%	✅

Minimum allowed coverage is 80%

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