Tumblr themes: Change the color and strength of overlay tints
When you hover a photo there is a block of color or 'overlay' that appears over the image. you can adjust this to any color and strength (opacity) that you want.
To start click the 'Edit theme' icon on the top right of your logged in home page to open the edit side panel.
Look for the following option panels:
Post overlay - this is a simple color picker where you can choose the color that appears when you hover an image
Post overlay strength - this changes the strength or opacity of the color you have chosen. Experiment with this until you get good legibility of icons and meta/tags.
Because the icons and meta data that appear over your posts are white try to keep your overlays fairly dark in color and opacity combination or people won't be able to read them.