This commit includes a new style sheet for MATE desktops. It isn't fully complete, but MATE panelbars and panelbar plugins should look much better. There's still work to be made here.
Made some adjustments to GTK3 entrybox buttons and comboboxes.
Changed the panelbar launcher button default icon scale to 24px (from 26px.)
Re-worked the INSTALL.md guide to include a table-of-contents and re-write some of the instructions too.
EMH-Mark-I