ruler: update colors when theme is changed

Merged Thejas Kiran P S requested to merge thejaskiranps/pitivi:color-cache-fix into master

Extracted out the lines where colors are set to separate function and connected it to 'theme changed' signal. (diff is a little weird try viewing side by side)

Fixes #2462 (closed)

Edited by Thejas Kiran P S

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