* Move color values to constant
* Fix MonacoPreview flickering on file selection and resizing
* Fix MarkdownPreview flickering on file selection and resizing
* Fix SvgPreview flickering on file selection and resizing
* Create Settings class and standardize background setting for MarkdownPreview
* Replace ColorTranslator.FromHtml with Color.FromArgb for constant color settings
* Use existing SetBackground
* Remove duplicate GetTheme function
* Update src/modules/previewpane/MarkdownPreviewHandler/Settings.cs